diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9258e5a3b6..05de113ee2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,7 @@
 * Initialize stylesheet with `/* empty */` to enable setting CSP directive that also works in Safari ([#7800](https://github.com/sveltejs/svelte/pull/7800))
 * Treat slots as if they don't exist when using CSS adjacent and general sibling combinators ([#8284](https://github.com/sveltejs/svelte/issues/8284))
 * Fix transitions so that they don't require a `style-src 'unsafe-inline'` Content Security Policy (CSP) ([#6662](https://github.com/sveltejs/svelte/issues/6662)).
+* Explicitly disallow `var` declarations extending the reactive statement scope ([#6800](https://github.com/sveltejs/svelte/pull/6800))
 
 ## Unreleased (3.0)
 
diff --git a/src/compiler/compile/Component.ts b/src/compiler/compile/Component.ts
index 2ca04f3adf..f6ef17f641 100644
--- a/src/compiler/compile/Component.ts
+++ b/src/compiler/compile/Component.ts
@@ -26,7 +26,7 @@ import TemplateScope from './nodes/shared/TemplateScope';
 import fuzzymatch from '../utils/fuzzymatch';
 import get_object from './utils/get_object';
 import Slot from './nodes/Slot';
-import { Node, ImportDeclaration, ExportNamedDeclaration, Identifier, ExpressionStatement, AssignmentExpression, Literal, Property, RestElement, ExportDefaultDeclaration, ExportAllDeclaration, FunctionDeclaration, FunctionExpression, ObjectExpression } from 'estree';
+import { Node, ImportDeclaration, ExportNamedDeclaration, Identifier, ExpressionStatement, AssignmentExpression, Literal, Property, RestElement, ExportDefaultDeclaration, ExportAllDeclaration, FunctionDeclaration, FunctionExpression, VariableDeclarator, ObjectExpression } from 'estree';
 import add_to_set from './utils/add_to_set';
 import check_graph_for_cycles from './utils/check_graph_for_cycles';
 import { print, b } from 'code-red';
@@ -1356,10 +1356,23 @@ export default class Component {
 				const module_dependencies = new Set<string>();
 
 				let scope = this.instance_scope;
+				const { declarations: outset_scope_decalarations } = this.instance_scope;
 				const map = this.instance_scope_map;
 
 				walk(node.body, {
 					enter(node: Node, parent) {
+						if (node.type === 'VariableDeclaration' && node.kind === 'var') {
+							const is_var_in_outset = node.declarations.some((declaration: VariableDeclarator) => {
+								const names: string[] = extract_names(declaration.id);
+								return !!names.find((name: string) => {
+									const var_node = outset_scope_decalarations.get(name);
+									return var_node === node;
+								});
+							});
+							if (is_var_in_outset) {
+								return component.error(node as any, compiler_errors.invalid_var_declaration);
+							}
+						}
 						if (map.has(node)) {
 							scope = map.get(node);
 						}
diff --git a/src/compiler/compile/compiler_errors.ts b/src/compiler/compile/compiler_errors.ts
index 860fa20d9f..6b891d2574 100644
--- a/src/compiler/compile/compiler_errors.ts
+++ b/src/compiler/compile/compiler_errors.ts
@@ -300,6 +300,10 @@ export default {
 		code: 'invalid-component-style-directive',
 		message: 'Style directives cannot be used on components'
 	},
+	invalid_var_declaration: {
+		code: 'invalid_var_declaration',
+		message: '"var" scope should not extend outside the reactive block'
+	},
 	invalid_style_directive_modifier: (valid: string) => ({
 		code: 'invalid-style-directive-modifier',
 		message: `Valid modifiers for style directives are: ${valid}`
diff --git a/test/validator/samples/invalid-reactive-var-1/errors.json b/test/validator/samples/invalid-reactive-var-1/errors.json
new file mode 100644
index 0000000000..9316d51168
--- /dev/null
+++ b/test/validator/samples/invalid-reactive-var-1/errors.json
@@ -0,0 +1,6 @@
+[{
+	"code": "invalid_var_declaration",
+	"message": "\"var\" scope should not extend outside the reactive block",
+  "start": { "line": 14, "column": 7 },
+  "end": { "line": 14, "column": 16 }
+}]
diff --git a/test/validator/samples/invalid-reactive-var-1/input.svelte b/test/validator/samples/invalid-reactive-var-1/input.svelte
new file mode 100644
index 0000000000..7fe774959d
--- /dev/null
+++ b/test/validator/samples/invalid-reactive-var-1/input.svelte
@@ -0,0 +1,20 @@
+<script>
+	var a;
+	var {a, b: [d, f], c}= {a: 1, b: [1, 2], c: 2};
+	$: {
+		function one() {
+			var a = 'a';
+			function two() {
+				var a = 'b';
+				return a;
+			}
+			return two();
+		}
+		a = one();
+		for (var i = 0; i<5; i ++ ) {
+			// Todo
+		}
+	}
+</script>
+
+<h1>Hello {a}</h1>
diff --git a/test/validator/samples/invalid-reactive-var-2/errors.json b/test/validator/samples/invalid-reactive-var-2/errors.json
new file mode 100644
index 0000000000..c28d767778
--- /dev/null
+++ b/test/validator/samples/invalid-reactive-var-2/errors.json
@@ -0,0 +1,6 @@
+[{
+	"code": "invalid_var_declaration",
+	"message": "\"var\" scope should not extend outside the reactive block",
+  "start": { "line": 4, "column": 2 },
+  "end": { "line": 4, "column": 50 }
+}]
diff --git a/test/validator/samples/invalid-reactive-var-2/input.svelte b/test/validator/samples/invalid-reactive-var-2/input.svelte
new file mode 100644
index 0000000000..b3a1c4272c
--- /dev/null
+++ b/test/validator/samples/invalid-reactive-var-2/input.svelte
@@ -0,0 +1,8 @@
+<script>
+	$: {
+		let f = 'f';
+		var {a, b: [c, d], e} = {a: 1, b: [2, 3], e: 4};
+	}
+</script>
+
+<h1>Hello</h1>
diff --git a/test/validator/samples/valid-reactive-vars/errors.json b/test/validator/samples/valid-reactive-vars/errors.json
new file mode 100644
index 0000000000..fe51488c70
--- /dev/null
+++ b/test/validator/samples/valid-reactive-vars/errors.json
@@ -0,0 +1 @@
+[]
diff --git a/test/validator/samples/valid-reactive-vars/input.svelte b/test/validator/samples/valid-reactive-vars/input.svelte
new file mode 100644
index 0000000000..0052101bd9
--- /dev/null
+++ b/test/validator/samples/valid-reactive-vars/input.svelte
@@ -0,0 +1,18 @@
+<script>
+  var name = 'name';
+  var c = 'cc';
+  let d = 'd';
+  $: {
+      function a() {
+        function b() {
+          var c = 'c';
+          return c;
+        }
+        return b;
+      }
+    let d = 'dd';
+    name = a()();
+  }
+</script>
+
+<h1>Hello {name}</h1>