diff --git a/.changeset/tidy-frogs-tease.md b/.changeset/tidy-frogs-tease.md
new file mode 100644
index 0000000000..b67f3a127d
--- /dev/null
+++ b/.changeset/tidy-frogs-tease.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: do not connect dependencies of an unconnected derived during init-time reads
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 227203523e..55a43ec2f8 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -660,12 +660,19 @@ export function get(signal) {
 		}
 
 		// connect disconnected deriveds if we are reading them inside an effect,
-		// or inside another derived that is already connected
+		// or inside another derived that is already connected. A derived reader
+		// that is itself unconnected must not connect its dependencies, even
+		// while an effect update is in progress (e.g. an init-time prop read
+		// evaluating a parent prop-expression memo): the dependency would be
+		// subscribed into the graph while nothing subscribes to the reader, so
+		// no destroy cascade could ever reach it (#18420)
 		var should_connect =
 			(derived.f & CONNECTED) === 0 &&
 			!untracking &&
 			active_reaction !== null &&
-			(is_updating_effect || (active_reaction.f & CONNECTED) !== 0);
+			((active_reaction.f & DERIVED) !== 0
+				? (active_reaction.f & CONNECTED) !== 0
+				: is_updating_effect || (active_reaction.f & CONNECTED) !== 0);
 
 		var is_new = (derived.f & REACTION_RAN) === 0;
 
diff --git a/packages/svelte/tests/signals/test.ts b/packages/svelte/tests/signals/test.ts
index 927ce2e665..6f0ad54dab 100644
--- a/packages/svelte/tests/signals/test.ts
+++ b/packages/svelte/tests/signals/test.ts
@@ -3,6 +3,7 @@ import { flushSync } from '../../src/index-client';
 import * as $ from '../../src/internal/client/runtime';
 import { push, pop } from '../../src/internal/client/context';
 import {
+	branch,
 	effect,
 	effect_root,
 	render_effect,
@@ -1503,4 +1504,52 @@ describe('signals', () => {
 			assert.deepEqual(log, ['inner destroyed', 'inner destroyed']);
 		};
 	});
+
+	// https://github.com/sveltejs/svelte/issues/18420
+	test('init-time read of an unconnected derived does not zombie-connect its dependencies', () => {
+		const s = state(0);
+		const inner = derived(() => $.get(s));
+		const memo = derived(() => $.get(inner));
+
+		return () => {
+			// mirrors a component's init-time prop read (e.g. a prop default):
+			// a branch effect body runs with `active_reaction === null` while an
+			// effect update is in progress, and the read evaluates the parent's
+			// prop-expression memo unconnected
+			const destroy = effect_root(() => {
+				branch(() => {
+					assert.equal($.get(memo), 0);
+				});
+			});
+
+			destroy();
+
+			// `inner` must not stay subscribed to `s` — nothing holds a
+			// reference to `inner` that a destroy cascade could reach
+			assert.equal(s.reactions, null);
+			assert.equal(inner.reactions, null);
+		};
+	});
+
+	// https://github.com/sveltejs/svelte/issues/18420
+	test('connected read of the same derived chain still cleans up on destroy', () => {
+		const s = state(0);
+		const inner = derived(() => $.get(s));
+		const memo = derived(() => $.get(inner));
+
+		return () => {
+			const destroy = effect_root(() => {
+				branch(() => {
+					render_effect(() => {
+						assert.equal($.get(memo), 0);
+					});
+				});
+			});
+
+			flushSync();
+			destroy();
+
+			assert.equal(s.reactions, null);
+		};
+	});
 });