diff --git a/src/lib/components/ImageLightbox.svelte b/src/lib/components/ImageLightbox.svelte
new file mode 100644
index 0000000..8369f2b
--- /dev/null
+++ b/src/lib/components/ImageLightbox.svelte
@@ -0,0 +1,49 @@
+<script lang="ts">
+	import XIcon from 'lucide-svelte/icons/x';
+
+	interface Props {
+		src: string;
+		alt: string;
+		onClose: () => void;
+	}
+
+	const { src, alt, onClose }: Props = $props();
+
+	function handleKeydown(e: KeyboardEvent) {
+		if (e.key === 'Escape') {
+			e.stopPropagation();
+			onClose();
+		}
+	}
+
+	function handleBackgroundClick(e: MouseEvent) {
+		if (e.target === e.currentTarget) {
+			onClose();
+		}
+	}
+</script>
+
+<svelte:window onkeydown={handleKeydown} />
+
+<!-- svelte-ignore a11y_no_static_element_interactions -->
+<div
+	class="fixed inset-0 z-50 flex items-center justify-center bg-black/80 animate-[fade-in_150ms_ease-out]"
+	onclick={handleBackgroundClick}
+	data-testid="image-lightbox"
+>
+	<button
+		onclick={onClose}
+		class="absolute right-4 top-4 rounded-sm p-2 text-white hover:bg-white/10"
+		aria-label="Close lightbox"
+		data-testid="lightbox-close"
+	>
+		<XIcon class="h-6 w-6" />
+	</button>
+
+	<img
+		{src}
+		{alt}
+		class="max-h-[90vh] max-w-[90vw] object-contain rounded-sm animate-[pop-in_150ms_ease-out]"
+		data-testid="lightbox-image"
+	/>
+</div>
diff --git a/src/lib/components/ImageUpload.svelte b/src/lib/components/ImageUpload.svelte
index 4cfecb6..ef9f18d 100644
--- a/src/lib/components/ImageUpload.svelte
+++ b/src/lib/components/ImageUpload.svelte
@@ -1,6 +1,7 @@
 <script lang="ts">
 	import XIcon from 'lucide-svelte/icons/x';
 	import RefreshCw from 'lucide-svelte/icons/refresh-cw';
+	import ImageLightbox from './ImageLightbox.svelte';
 	import type { Attachment } from '$lib/types/index.js';
 	import { optimizeImage } from '$lib/utils/image-optimize.js';
 	import { browser } from '$app/environment';
@@ -18,6 +19,8 @@
 	let uploading = $state(false);
 	let optimizing = $state(false);
 	let dragOver = $state(false);
+	let lightboxSrc = $state<string | null>(null);
+	let lightboxAlt = $state('');
 
 	async function handleFiles(files: FileList | null) {
 		if (!files || !noteId) return;
@@ -108,10 +111,12 @@
 				{@const isPending = 'pending' in attachment && attachment.pending}
 				{@const imgSrc = isPending ? attachment.path : `/api/notes/${noteId}/attachments?attachmentId=${attachment.id}`}
 				<div class="group relative">
+					<!-- svelte-ignore a11y_no_noninteractive_element_interactions a11y_click_events_have_key_events -->
 					<img
 						src={imgSrc}
 						alt={attachment.filename}
-						class="h-20 w-20 rounded-sm object-cover {isPending ? 'opacity-70' : ''}"
+						class="h-20 w-20 cursor-pointer rounded-sm object-cover {isPending ? 'opacity-70' : ''}"
+						onclick={() => { lightboxSrc = imgSrc; lightboxAlt = attachment.filename; }}
 						data-testid="attachment-thumbnail"
 					/>
 					{#if isPending}
@@ -161,3 +166,7 @@
 		{/if}
 	</div>
 </div>
+
+{#if lightboxSrc}
+	<ImageLightbox src={lightboxSrc} alt={lightboxAlt} onClose={() => lightboxSrc = null} />
+{/if}
diff --git a/src/lib/components/NoteCard.svelte b/src/lib/components/NoteCard.svelte
index baecf0b..0ad4195 100644
--- a/src/lib/components/NoteCard.svelte
+++ b/src/lib/components/NoteCard.svelte
@@ -3,6 +3,7 @@
 	import { renderMarkdown } from '$lib/utils/markdown.js';
 	import { togglePin, trashNote, archiveNote, unarchiveNote, restoreNote, deleteNote, updateNote, currentFilter } from '$lib/stores/notes.js';
 	import TagChip from './TagChip.svelte';
+	import ImageLightbox from './ImageLightbox.svelte';
 	import type { Note } from '$lib/types/index.js';
 	import Undo2 from 'lucide-svelte/icons/undo-2';
 	import Trash2 from 'lucide-svelte/icons/trash-2';
@@ -29,6 +30,8 @@
 	});
 
 	let cardStyle = $state('');
+	let lightboxSrc = $state<string | null>(null);
+	let lightboxAlt = $state('');
 
 	const renderedContent = $derived(renderMarkdown(note.content));
 
@@ -83,11 +86,13 @@
 		<div class="-mx-4 -mt-4 mb-3 flex overflow-hidden rounded-t-sm" data-testid="card-thumbnails">
 			{#each note.attachments.slice(0, 3) as attachment}
 				<div class="relative min-w-0 flex-1">
+					<!-- svelte-ignore a11y_no_noninteractive_element_interactions a11y_click_events_have_key_events -->
 					<img
 						src="/api/notes/{note.id}/attachments?attachmentId={attachment.id}&thumb=1"
 						alt={attachment.filename}
-						class="h-24 w-full object-cover"
+						class="h-24 w-full cursor-pointer object-cover"
 						loading="lazy"
+						onclick={(e) => { e.stopPropagation(); lightboxSrc = `/api/notes/${note.id}/attachments?attachmentId=${attachment.id}`; lightboxAlt = attachment.filename; }}
 						data-testid="card-thumbnail"
 					/>
 				</div>
@@ -207,6 +212,9 @@
 		{/if}
 	</div>
 
+	{#if lightboxSrc}
+		<ImageLightbox src={lightboxSrc} alt={lightboxAlt} onClose={() => lightboxSrc = null} />
+	{/if}
 </article>
 
 <style>
diff --git a/src/lib/components/NoteEditor.svelte b/src/lib/components/NoteEditor.svelte
index bfc432f..33fc07e 100644
--- a/src/lib/components/NoteEditor.svelte
+++ b/src/lib/components/NoteEditor.svelte
@@ -4,6 +4,7 @@
 	import FormattingToolbar from './FormattingToolbar.svelte';
 	import TiptapEditor from './TiptapEditor.svelte';
 	import ImageUpload from './ImageUpload.svelte';
+	import ImageLightbox from './ImageLightbox.svelte';
 	import { updateNote, createNote } from '$lib/stores/notes.js';
 	import { NOTE_COLORS } from '$lib/utils/colors.js';
 	import type { Editor } from '@tiptap/core';
@@ -37,6 +38,8 @@
 	let textareaEl: HTMLTextAreaElement | undefined = $state();
 	let tiptapEditor: Editor | undefined = $state();
 	let editorTick = $state(0);
+	let lightboxSrc = $state<string | null>(null);
+	let lightboxAlt = $state('');
 
 	// Mutable note identity — allows transitioning from new → saved without closing
 	// svelte-ignore state_referenced_locally
@@ -203,16 +206,23 @@
 			<div class="border-t border-[var(--border-subtle)] px-4 py-2">
 				<div class="flex flex-wrap gap-2">
 					{#each attachmentsList as attachment}
+						<!-- svelte-ignore a11y_no_noninteractive_element_interactions a11y_click_events_have_key_events -->
 						<img
 							src="/api/notes/{noteId}/attachments?attachmentId={attachment.id}"
 							alt={attachment.filename}
-							class="h-20 w-20 rounded-sm object-cover"
+							class="h-20 w-20 cursor-pointer rounded-sm object-cover"
+							onclick={() => { lightboxSrc = `/api/notes/${noteId}/attachments?attachmentId=${attachment.id}`; lightboxAlt = attachment.filename; }}
+							data-testid="attachment-thumbnail"
 						/>
 					{/each}
 				</div>
 			</div>
 		{/if}
 
+		{#if lightboxSrc}
+			<ImageLightbox src={lightboxSrc} alt={lightboxAlt} onClose={() => lightboxSrc = null} />
+		{/if}
+
 		<!-- Formatting toolbar -->
 		{#if !rawMarkdownMode && !checklistMode}
 			<FormattingToolbar editor={tiptapEditor} tick={editorTick} />
diff --git a/tests/e2e/attachments.spec.ts b/tests/e2e/attachments.spec.ts
index a35f1fb..12800f9 100644
--- a/tests/e2e/attachments.spec.ts
+++ b/tests/e2e/attachments.spec.ts
@@ -84,6 +84,28 @@ test.describe('Image Attachments', () => {
 		await expect(card.getByTestId('card-thumbnail')).not.toBeVisible();
 	});
 
+	test('Scenario: Clicking an image thumbnail opens a full-size lightbox', async ({ authenticatedPage: page }) => {
+		// Given a note with an uploaded image exists
+		await createAndReopenNote(page, 'Lightbox Test');
+		await page.getByTestId('image-toggle').click();
+		await page.getByTestId('file-input').setInputFiles(TEST_IMAGE_PATH);
+		await expect(page.getByTestId('attachment-thumbnail').first()).toBeVisible();
+
+		// When the user clicks the thumbnail
+		await page.getByTestId('attachment-thumbnail').first().click();
+
+		// Then a full-size lightbox overlay is displayed
+		await expect(page.getByTestId('image-lightbox')).toBeVisible();
+		await expect(page.getByTestId('lightbox-image')).toBeVisible();
+
+		// When the user presses Escape
+		await page.keyboard.press('Escape');
+
+		// Then the lightbox closes but the editor remains open
+		await expect(page.getByTestId('image-lightbox')).not.toBeVisible();
+		await expect(page.getByTestId('note-editor')).toBeVisible();
+	});
+
 	test('Scenario: Image toggle on new note auto-saves and opens upload panel', async ({ authenticatedPage: page }) => {
 		// When the user opens a new note and clicks the image toggle
 		await page.getByTestId('new-note-btn').click();