diff --git a/packages/svelte/src/compiler/phases/2-analyze/index.js b/packages/svelte/src/compiler/phases/2-analyze/index.js
index 51180ca7cc..fded183b86 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/index.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/index.js
@@ -48,6 +48,7 @@ import { Literal } from './visitors/Literal.js';
 import { MemberExpression } from './visitors/MemberExpression.js';
 import { NewExpression } from './visitors/NewExpression.js';
 import { OnDirective } from './visitors/OnDirective.js';
+import { PropertyDefinition } from './visitors/PropertyDefinition.js';
 import { RegularElement } from './visitors/RegularElement.js';
 import { RenderTag } from './visitors/RenderTag.js';
 import { SlotElement } from './visitors/SlotElement.js';
@@ -164,6 +165,7 @@ const visitors = {
 	MemberExpression,
 	NewExpression,
 	OnDirective,
+	PropertyDefinition,
 	RegularElement,
 	RenderTag,
 	SlotElement,
@@ -266,7 +268,7 @@ export function analyze_module(ast, options) {
 			scope,
 			scopes,
 			analysis: /** @type {ComponentAnalysis} */ (analysis),
-			state_fields: null,
+			state_fields: new Map(),
 			// TODO the following are not needed for modules, but we have to pass them in order to avoid type error,
 			// and reducing the type would result in a lot of tedious type casts elsewhere - find a good solution one day
 			ast_type: /** @type {any} */ (null),
@@ -626,7 +628,7 @@ export function analyze_component(root, source, options) {
 				has_props_rune: false,
 				component_slots: new Set(),
 				expression: null,
-				state_fields: null,
+				state_fields: new Map(),
 				function_depth: scope.function_depth,
 				reactive_statement: null
 			};
@@ -693,7 +695,7 @@ export function analyze_component(root, source, options) {
 				reactive_statement: null,
 				component_slots: new Set(),
 				expression: null,
-				state_fields: null,
+				state_fields: new Map(),
 				function_depth: scope.function_depth
 			};
 
diff --git a/packages/svelte/src/compiler/phases/2-analyze/types.d.ts b/packages/svelte/src/compiler/phases/2-analyze/types.d.ts
index 7ccd175ff3..080239bac0 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/types.d.ts
+++ b/packages/svelte/src/compiler/phases/2-analyze/types.d.ts
@@ -20,7 +20,7 @@ export interface AnalysisState {
 	expression: ExpressionMetadata | null;
 
 	/** Used to analyze class state */
-	state_fields: Record<string, StateField> | null;
+	state_fields: Map<string, StateField>;
 
 	function_depth: number;
 
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/ClassBody.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/ClassBody.js
index 224e4445ce..ffc39ac00d 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/ClassBody.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/ClassBody.js
@@ -30,14 +30,11 @@ export function ClassBody(node, context) {
 		}
 	}
 
-	/** @type {Record<string, StateField>} */
-	const state_fields = {};
+	/** @type {Map<string, StateField>} */
+	const state_fields = new Map();
 
 	context.state.analysis.classes.set(node, state_fields);
 
-	/** @type {string[]} */
-	const seen = [];
-
 	/** @type {MethodDefinition | null} */
 	let constructor = null;
 
@@ -53,24 +50,22 @@ export function ClassBody(node, context) {
 		const rune = get_rune(value, context.state.scope);
 
 		if (rune && is_state_creation_rune(rune)) {
-			if (seen.includes(name)) {
+			if (state_fields.has(name)) {
 				e.state_field_duplicate(node, name);
 			}
 
-			state_fields[name] = {
+			state_fields.set(name, {
 				node,
 				type: rune,
 				// @ts-expect-error for public state this is filled out in a moment
 				key: key.type === 'PrivateIdentifier' ? key : null,
 				value: /** @type {CallExpression} */ (value)
-			};
-
-			seen.push(name);
+			});
 		}
 	}
 
 	for (const child of node.body) {
-		if (child.type === 'PropertyDefinition' && !child.computed) {
+		if (child.type === 'PropertyDefinition' && !child.computed && !child.static) {
 			handle(child, child.key, child.value);
 		}
 
@@ -94,13 +89,11 @@ export function ClassBody(node, context) {
 		}
 	}
 
-	for (const name in state_fields) {
+	for (const [name, field] of state_fields) {
 		if (name[0] === '#') {
 			continue;
 		}
 
-		const field = state_fields[name];
-
 		let deconflicted = name.replace(regex_invalid_identifier_chars, '_');
 		while (private_ids.includes(deconflicted)) {
 			deconflicted = '_' + deconflicted;
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/PropertyDefinition.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/PropertyDefinition.js
new file mode 100644
index 0000000000..99d05cb47c
--- /dev/null
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/PropertyDefinition.js
@@ -0,0 +1,21 @@
+/** @import { PropertyDefinition } from 'estree' */
+/** @import { Context } from '../types' */
+import * as e from '../../../errors.js';
+import { get_name } from '../../nodes.js';
+
+/**
+ * @param {PropertyDefinition} node
+ * @param {Context} context
+ */
+export function PropertyDefinition(node, context) {
+	const name = get_name(node.key);
+	const field = name && context.state.state_fields.get(name);
+
+	if (field && node !== field.node && node.value) {
+		if (/** @type {number} */ (node.start) < /** @type {number} */ (field.node.start)) {
+			e.state_field_invalid_assignment(node);
+		}
+	}
+
+	context.next();
+}
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/shared/utils.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/shared/utils.js
index 89921832ae..10468595a4 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/shared/utils.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/shared/utils.js
@@ -42,11 +42,7 @@ export function validate_assignment(node, argument, context) {
 				? null
 				: get_name(argument.property);
 
-		const field =
-			name !== null &&
-			context.state.state_fields &&
-			Object.hasOwn(context.state.state_fields, name) &&
-			context.state.state_fields[name];
+		const field = name !== null && context.state.state_fields?.get(name);
 
 		// check we're not assigning to a state field before its declaration in the constructor
 		if (field && field.node.type === 'AssignmentExpression' && node !== field.node) {
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
index 711c1e64d8..f2eda3a7d2 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
@@ -163,7 +163,7 @@ export function client_component(analysis, options) {
 		},
 		events: new Set(),
 		preserve_whitespace: options.preserveWhitespace,
-		state_fields: {},
+		state_fields: new Map(),
 		transform: {},
 		in_constructor: false,
 		instance_level_snippets: [],
@@ -670,7 +670,7 @@ export function client_module(analysis, options) {
 		options,
 		scope: analysis.module.scope,
 		scopes: analysis.module.scopes,
-		state_fields: {},
+		state_fields: new Map(),
 		transform: {},
 		in_constructor: false
 	};
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js
index 2e0d728a03..a46b318601 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js
@@ -54,12 +54,11 @@ const callees = {
 function build_assignment(operator, left, right, context) {
 	if (context.state.analysis.runes && left.type === 'MemberExpression') {
 		const name = get_name(left.property);
+		const field = name && context.state.state_fields.get(name);
 
-		if (name !== null) {
+		if (field) {
 			// special case — state declaration in class constructor
-			const ancestor = context.path.at(-4);
-
-			if (ancestor?.type === 'MethodDefinition' && ancestor.kind === 'constructor') {
+			if (field.node.type === 'AssignmentExpression' && left === field.node.left) {
 				const rune = get_rune(right, context.state.scope);
 
 				if (rune) {
@@ -70,7 +69,7 @@ function build_assignment(operator, left, right, context) {
 
 					return b.assignment(
 						operator,
-						b.member(b.this, context.state.state_fields[name].key),
+						b.member(b.this, field.key),
 						/** @type {Expression} */ (context.visit(right, child_state))
 					);
 				}
@@ -78,20 +77,16 @@ function build_assignment(operator, left, right, context) {
 
 			// special case — assignment to private state field
 			if (left.property.type === 'PrivateIdentifier') {
-				const field = context.state.state_fields[name];
-
-				if (field) {
-					let value = /** @type {Expression} */ (
-						context.visit(build_assignment_value(operator, left, right))
-					);
+				let value = /** @type {Expression} */ (
+					context.visit(build_assignment_value(operator, left, right))
+				);
 
-					const needs_proxy =
-						field.type === '$state' &&
-						is_non_coercive_operator(operator) &&
-						should_proxy(value, context.state.scope);
+				const needs_proxy =
+					field.type === '$state' &&
+					is_non_coercive_operator(operator) &&
+					should_proxy(value, context.state.scope);
 
-					return b.call('$.set', left, value, needs_proxy && b.true);
-				}
+				return b.call('$.set', left, value, needs_proxy && b.true);
 			}
 		}
 	}
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/ClassBody.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/ClassBody.js
index 12ed7ccea5..71a8fc3d05 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/ClassBody.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/ClassBody.js
@@ -1,4 +1,5 @@
 /** @import { CallExpression, ClassBody, MethodDefinition, PropertyDefinition, StaticBlock } from 'estree' */
+/** @import { StateField } from '#compiler' */
 /** @import { Context } from '../types' */
 import * as b from '#compiler/builders';
 import { get_name } from '../../../nodes.js';
@@ -21,13 +22,11 @@ export function ClassBody(node, context) {
 
 	const child_state = { ...context.state, state_fields };
 
-	for (const name in state_fields) {
+	for (const [name, field] of state_fields) {
 		if (name[0] === '#') {
 			continue;
 		}
 
-		const field = state_fields[name];
-
 		// insert backing fields for stuff declared in the constructor
 		if (field.node.type === 'AssignmentExpression') {
 			const member = b.member(b.this, field.key);
@@ -61,13 +60,13 @@ export function ClassBody(node, context) {
 		}
 
 		const name = get_name(definition.key);
-		if (name === null || !Object.hasOwn(state_fields, name)) {
+		const field = name && /** @type {StateField} */ (state_fields.get(name));
+
+		if (!field) {
 			body.push(/** @type {PropertyDefinition} */ (context.visit(definition, child_state)));
 			continue;
 		}
 
-		const field = state_fields[name];
-
 		if (name[0] === '#') {
 			body.push(/** @type {PropertyDefinition} */ (context.visit(definition, child_state)));
 		} else if (field.node === definition) {
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/MemberExpression.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/MemberExpression.js
index ded727dc0f..7e89b0e705 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/MemberExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/MemberExpression.js
@@ -9,7 +9,7 @@ import * as b from '#compiler/builders';
 export function MemberExpression(node, context) {
 	// rewrite `this.#foo` as `this.#foo.v` inside a constructor
 	if (node.property.type === 'PrivateIdentifier') {
-		const field = context.state.state_fields['#' + node.property.name];
+		const field = context.state.state_fields.get('#' + node.property.name);
 
 		if (field) {
 			return context.state.in_constructor &&
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/UpdateExpression.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/UpdateExpression.js
index c3c9db6270..66952eb298 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/UpdateExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/UpdateExpression.js
@@ -15,7 +15,7 @@ export function UpdateExpression(node, context) {
 		argument.type === 'MemberExpression' &&
 		argument.object.type === 'ThisExpression' &&
 		argument.property.type === 'PrivateIdentifier' &&
-		Object.hasOwn(context.state.state_fields, '#' + argument.property.name)
+		context.state.state_fields.has('#' + argument.property.name)
 	) {
 		let fn = '$.update';
 		if (node.prefix) fn += '_pre';
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/transform-server.js b/packages/svelte/src/compiler/phases/3-transform/server/transform-server.js
index 2c677116dc..3498b2636b 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/transform-server.js
+++ b/packages/svelte/src/compiler/phases/3-transform/server/transform-server.js
@@ -97,7 +97,7 @@ export function server_component(analysis, options) {
 		template: /** @type {any} */ (null),
 		namespace: options.namespace,
 		preserve_whitespace: options.preserveWhitespace,
-		state_fields: {},
+		state_fields: new Map(),
 		skip_hydration_boundaries: false
 	};
 
@@ -393,7 +393,7 @@ export function server_module(analysis, options) {
 		// to be present for `javascript_visitors_legacy` and so is included in module
 		// transform state as well as component transform state
 		legacy_reactive_statements: new Map(),
-		state_fields: {}
+		state_fields: new Map()
 	};
 
 	const module = /** @type {Program} */ (
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/AssignmentExpression.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/AssignmentExpression.js
index c10e757d73..280c16dbd2 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/visitors/AssignmentExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/server/visitors/AssignmentExpression.js
@@ -26,26 +26,23 @@ export function AssignmentExpression(node, context) {
 function build_assignment(operator, left, right, context) {
 	if (context.state.analysis.runes && left.type === 'MemberExpression') {
 		const name = get_name(left.property);
+		const field = name && context.state.state_fields.get(name);
 
-		if (name !== null) {
-			// special case — state declaration in class constructor
-			const ancestor = context.path.at(-4);
+		// special case — state declaration in class constructor
+		if (field && field.node.type === 'AssignmentExpression' && left === field.node.left) {
+			const rune = get_rune(right, context.state.scope);
 
-			if (ancestor?.type === 'MethodDefinition' && ancestor.kind === 'constructor') {
-				const rune = get_rune(right, context.state.scope);
+			if (rune) {
+				const key =
+					left.property.type === 'PrivateIdentifier' || rune === '$state' || rune === '$state.raw'
+						? left.property
+						: field.key;
 
-				if (rune) {
-					const key =
-						left.property.type === 'PrivateIdentifier' || rune === '$state' || rune === '$state.raw'
-							? left.property
-							: context.state.state_fields[name].key;
-
-					return b.assignment(
-						operator,
-						b.member(b.this, key, key.type === 'Literal'),
-						/** @type {Expression} */ (context.visit(right))
-					);
-				}
+				return b.assignment(
+					operator,
+					b.member(b.this, key, key.type === 'Literal'),
+					/** @type {Expression} */ (context.visit(right))
+				);
 			}
 		}
 	}
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/ClassBody.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/ClassBody.js
index 45eddc424c..6797b0beff 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/visitors/ClassBody.js
+++ b/packages/svelte/src/compiler/phases/3-transform/server/visitors/ClassBody.js
@@ -21,15 +21,14 @@ export function ClassBody(node, context) {
 
 	const child_state = { ...context.state, state_fields };
 
-	for (const name in state_fields) {
+	for (const [name, field] of state_fields) {
 		if (name[0] === '#') {
 			continue;
 		}
 
-		const field = state_fields[name];
-
 		// insert backing fields for stuff declared in the constructor
 		if (
+			field &&
 			field.node.type === 'AssignmentExpression' &&
 			(field.type === '$derived' || field.type === '$derived.by')
 		) {
@@ -52,13 +51,13 @@ export function ClassBody(node, context) {
 		}
 
 		const name = get_name(definition.key);
-		if (name === null || !Object.hasOwn(state_fields, name)) {
+		const field = name && state_fields.get(name);
+
+		if (!field) {
 			body.push(/** @type {PropertyDefinition} */ (context.visit(definition, child_state)));
 			continue;
 		}
 
-		const field = state_fields[name];
-
 		if (name[0] === '#' || field.type === '$state' || field.type === '$state.raw') {
 			body.push(/** @type {PropertyDefinition} */ (context.visit(definition, child_state)));
 		} else if (field.node === definition) {
diff --git a/packages/svelte/src/compiler/phases/3-transform/types.d.ts b/packages/svelte/src/compiler/phases/3-transform/types.d.ts
index d7695a60ea..887ad447b9 100644
--- a/packages/svelte/src/compiler/phases/3-transform/types.d.ts
+++ b/packages/svelte/src/compiler/phases/3-transform/types.d.ts
@@ -8,5 +8,5 @@ export interface TransformState {
 	readonly scope: Scope;
 	readonly scopes: Map<AST.SvelteNode, Scope>;
 
-	readonly state_fields: Record<string, StateField>;
+	readonly state_fields: Map<string, StateField>;
 }
diff --git a/packages/svelte/src/compiler/phases/types.d.ts b/packages/svelte/src/compiler/phases/types.d.ts
index 6e3aa9e026..67cbd75ff8 100644
--- a/packages/svelte/src/compiler/phases/types.d.ts
+++ b/packages/svelte/src/compiler/phases/types.d.ts
@@ -38,7 +38,7 @@ export interface Analysis {
 	immutable: boolean;
 	tracing: boolean;
 
-	classes: Map<ClassBody, Record<string, StateField>>;
+	classes: Map<ClassBody, Map<string, StateField>>;
 
 	// TODO figure out if we can move this to ComponentAnalysis
 	accessors: boolean;
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-9/errors.json b/packages/svelte/tests/validator/samples/class-state-constructor-9/errors.json
new file mode 100644
index 0000000000..b7dd4c8ed4
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-9/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_field_invalid_assignment",
+		"message": "Cannot assign to a state field before its declaration",
+		"start": {
+			"line": 2,
+			"column": 1
+		},
+		"end": {
+			"line": 2,
+			"column": 12
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-9/input.svelte.js b/packages/svelte/tests/validator/samples/class-state-constructor-9/input.svelte.js
new file mode 100644
index 0000000000..a8469e13af
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-9/input.svelte.js
@@ -0,0 +1,7 @@
+export class Counter {
+	count = -1;
+
+	constructor() {
+		this.count = $state(0);
+	}
+}