diff --git a/CHANGELOG.md b/CHANGELOG.md
index edf146e..fc13022 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -35,6 +35,7 @@
 - font size selection
 - line spacing button
 - text alignment
+- show formatting marks
 
 ### Not yet implemented
 - ODT import / open existing file
diff --git a/src/App.svelte b/src/App.svelte
index 33200da..cdff7a0 100644
--- a/src/App.svelte
+++ b/src/App.svelte
@@ -5,7 +5,7 @@
   import ToolbarExpanded from './lib/editor/ToolbarExpanded.svelte';
   import { exportToOdt } from './lib/export/odt';
   import { getPageBreakDebug } from './lib/editor/pageBreaks';
-  import { loadTheme, saveTheme, applyTheme, loadToolbarExpanded, saveToolbarExpanded, type ThemeMode } from './lib/storage/theme';
+  import { loadTheme, saveTheme, applyTheme, loadToolbarExpanded, saveToolbarExpanded, loadFormattingMarks, saveFormattingMarks, type ThemeMode } from './lib/storage/theme';
 
   let editor: Editor | null = $state(null);
   let tick: number = $state(0);
@@ -15,8 +15,13 @@
   let themeMode: ThemeMode = $state(loadTheme());
   let themeOpen = $state(false);
   let toolbarExpanded = $state(loadToolbarExpanded());
+  let showFormattingMarks = $state(loadFormattingMarks());
   let zoom = $state(Math.max(50, Math.min(200, parseInt(localStorage.getItem('odf-editor-zoom') ?? '100', 10))));
 
+  $effect(() => {
+    saveFormattingMarks(showFormattingMarks);
+  });
+
   function setZoom(value: number) {
     zoom = Math.max(50, Math.min(200, value));
     localStorage.setItem('odf-editor-zoom', String(zoom));
@@ -145,9 +150,9 @@
     </div>
   </header>
   {#if toolbarExpanded}
-    <ToolbarExpanded {editor} {tick} />
+    <ToolbarExpanded {editor} {tick} bind:showFormattingMarks />
   {/if}
-  <EditorComponent bind:editor bind:tick bind:currentPage bind:numPages {zoom} />
+  <EditorComponent bind:editor bind:tick bind:currentPage bind:numPages {zoom} {showFormattingMarks} />
   <footer class="statusbar">
     <span>Page {currentPage} of {numPages}</span>
     <div class="zoom-controls">
diff --git a/src/lib/editor/Editor.svelte b/src/lib/editor/Editor.svelte
index c6ae695..7a91153 100644
--- a/src/lib/editor/Editor.svelte
+++ b/src/lib/editor/Editor.svelte
@@ -9,8 +9,8 @@
 
   const DEFAULT_EDITOR_FONT = 'Georgia'; // must match ToolbarExpanded.svelte
 
-  let { editor = $bindable(), tick = $bindable(0), currentPage = $bindable(1), numPages = $bindable(1), zoom = 100 }: {
-    editor: Editor | null; tick: number; currentPage: number; numPages: number; zoom: number;
+  let { editor = $bindable(), tick = $bindable(0), currentPage = $bindable(1), numPages = $bindable(1), zoom = 100, showFormattingMarks = false }: {
+    editor: Editor | null; tick: number; currentPage: number; numPages: number; zoom: number; showFormattingMarks?: boolean;
   } = $props();
 
   let element: HTMLDivElement;
@@ -119,5 +119,5 @@
 </script>
 
 <div class="editor" bind:this={editorContainer}>
-  <div bind:this={element} class="paper" style="zoom: {zoom / 100}"></div>
+  <div bind:this={element} class="paper" class:show-formatting-marks={showFormattingMarks} style="zoom: {zoom / 100}"></div>
 </div>
diff --git a/src/lib/editor/ToolbarExpanded.svelte b/src/lib/editor/ToolbarExpanded.svelte
index 5fe5c94..d141be9 100644
--- a/src/lib/editor/ToolbarExpanded.svelte
+++ b/src/lib/editor/ToolbarExpanded.svelte
@@ -1,7 +1,7 @@
 <script lang="ts">
   import type { Editor } from '@tiptap/core';
 
-  let { editor, tick }: { editor: Editor | null; tick: number } = $props();
+  let { editor, tick, showFormattingMarks = $bindable() }: { editor: Editor | null; tick: number; showFormattingMarks: boolean } = $props();
 
   type AlignValue = 'left' | 'center' | 'right' | 'justify';
 
@@ -459,6 +459,21 @@
 
     <div class="toolbar-separator"></div>
 
+    <div class="toolbar-group">
+      <button
+        class:active={showFormattingMarks}
+        onclick={() => (showFormattingMarks = !showFormattingMarks)}
+        title="Formatting marks"
+        aria-pressed={showFormattingMarks}
+      >
+        <svg width="16" height="16" viewBox="0 0 16 16" fill="none" aria-hidden="true">
+          <path d="M11 2.25v11.5M7.5 2.25v11.5M11 2.25H6.75a2.75 2.75 0 0 0 0 5.5H7.5" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+        </svg>
+      </button>
+    </div>
+
+    <div class="toolbar-separator"></div>
+
     <div class="toolbar-group">
       <button disabled title="Insert table (coming soon)">
         <svg width="16" height="16" viewBox="0 0 16 16" fill="none" aria-hidden="true">
diff --git a/src/lib/editor/extensions.ts b/src/lib/editor/extensions.ts
index a2fc690..03dbf3f 100644
--- a/src/lib/editor/extensions.ts
+++ b/src/lib/editor/extensions.ts
@@ -15,6 +15,7 @@ import Placeholder from '@tiptap/extension-placeholder';
 import TextAlign from '@tiptap/extension-text-align';
 import { PageBreaks } from './pageBreaks';
 import { LineHeight } from './lineHeight';
+import { FormattingMarks } from './formattingMarks';
 
 export const extensions = [
   Document,
@@ -40,4 +41,5 @@ export const extensions = [
     defaultAlignment: 'left',
   }),
   PageBreaks,
+  FormattingMarks,
 ];
diff --git a/src/lib/editor/formattingMarks.ts b/src/lib/editor/formattingMarks.ts
new file mode 100644
index 0000000..242ae25
--- /dev/null
+++ b/src/lib/editor/formattingMarks.ts
@@ -0,0 +1,95 @@
+import { Extension } from '@tiptap/core';
+import { Plugin, PluginKey } from '@tiptap/pm/state';
+import { Decoration, DecorationSet } from '@tiptap/pm/view';
+import type { EditorState, Transaction } from '@tiptap/pm/state';
+import type { Node as PmNode } from '@tiptap/pm/model';
+
+const formattingMarksKey = new PluginKey<DecorationSet>('formattingMarks');
+
+const FORCE_REBUILD_META = 'formattingMarks/forceRebuild';
+
+function isEnabled(domRoot: HTMLElement | null): boolean {
+  return !!domRoot?.closest('.paper')?.classList.contains('show-formatting-marks');
+}
+
+function buildDecorations(doc: PmNode): DecorationSet {
+  const decos: Decoration[] = [];
+  doc.descendants((node, pos) => {
+    if (!node.isText) return;
+    const text = node.text ?? '';
+    for (let i = 0; i < text.length; i++) {
+      if (text.charCodeAt(i) === 32) {
+        const from = pos + i;
+        decos.push(Decoration.inline(from, from + 1, { class: 'pm-space-mark' }));
+      }
+    }
+  });
+  return decos.length ? DecorationSet.create(doc, decos) : DecorationSet.empty;
+}
+
+export const FormattingMarks = Extension.create({
+  name: 'formattingMarks',
+
+  addProseMirrorPlugins() {
+    return [
+      new Plugin<DecorationSet>({
+        key: formattingMarksKey,
+        state: {
+          init: () => DecorationSet.empty,
+          apply(tr: Transaction, old: DecorationSet, _oldState: EditorState, newState: EditorState) {
+            const force = tr.getMeta(FORCE_REBUILD_META) === true;
+            // Resolve enabled-state via the DOM class on .paper. The plugin's
+            // `view` hook below installs a MutationObserver that dispatches a
+            // FORCE_REBUILD meta whenever that class toggles, so this check
+            // always reflects the current UI state.
+            // During init (no view yet) we can't read DOM, so default to keeping
+            // the previous set mapped through.
+            if (typeof document === 'undefined') {
+              return tr.docChanged ? old.map(tr.mapping, tr.doc) : old;
+            }
+            const enabled = document.querySelector('.paper.show-formatting-marks') !== null;
+            if (!enabled) return DecorationSet.empty;
+            if (force || tr.docChanged) return buildDecorations(newState.doc);
+            return old;
+          },
+        },
+        props: {
+          decorations(state) {
+            return formattingMarksKey.getState(state);
+          },
+        },
+        view(editorView) {
+          const paper = editorView.dom.closest('.paper') as HTMLElement | null;
+          let observer: MutationObserver | null = null;
+
+          const triggerRebuild = () => {
+            editorView.dispatch(editorView.state.tr.setMeta(FORCE_REBUILD_META, true));
+          };
+
+          if (paper) {
+            observer = new MutationObserver((mutations) => {
+              for (const m of mutations) {
+                if (m.type === 'attributes' && m.attributeName === 'class') {
+                  triggerRebuild();
+                  break;
+                }
+              }
+            });
+            observer.observe(paper, { attributes: true, attributeFilter: ['class'] });
+          }
+
+          // Initial pass: if the class is already on at mount-time, render now.
+          if (isEnabled(editorView.dom)) {
+            queueMicrotask(triggerRebuild);
+          }
+
+          return {
+            destroy() {
+              observer?.disconnect();
+            },
+          };
+        },
+      }),
+    ];
+  },
+});
diff --git a/src/lib/storage/theme.ts b/src/lib/storage/theme.ts
index 114798d..2b78d79 100644
--- a/src/lib/storage/theme.ts
+++ b/src/lib/storage/theme.ts
@@ -2,6 +2,7 @@ export type ThemeMode = 'light' | 'dark' | 'auto';
 
 const THEME_KEY = 'odf-editor-theme';
 const TOOLBAR_KEY = 'odf-editor-toolbar-expanded';
+const FORMATTING_MARKS_KEY = 'odf-editor-formatting-marks';
 
 export function loadTheme(): ThemeMode {
     const saved = localStorage.getItem(THEME_KEY);
@@ -21,6 +22,14 @@ export function saveToolbarExpanded(expanded: boolean): void {
     localStorage.setItem(TOOLBAR_KEY, String(expanded));
 }
 
+export function loadFormattingMarks(): boolean {
+    return localStorage.getItem(FORMATTING_MARKS_KEY) === 'true';
+}
+
+export function saveFormattingMarks(enabled: boolean): void {
+    localStorage.setItem(FORMATTING_MARKS_KEY, String(enabled));
+}
+
 function resolveMode(mode: ThemeMode): 'light' | 'dark' {
   if (mode === 'auto') {
     return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
diff --git a/src/styles/editor.css b/src/styles/editor.css
index 397059c..4d1db81 100644
--- a/src/styles/editor.css
+++ b/src/styles/editor.css
@@ -156,6 +156,73 @@
   height: 0;
 }
 
+/* Formatting marks (Toggle in extended toolbar).
+   Zero-width inline-block with visible overflow: the ¶ glyph renders just
+   past the last character without consuming line width — so right/center/
+   justify alignment of the actual text is preserved. The leading NBSP in
+   the content gives a small visual gap from the text. */
+.paper.show-formatting-marks .tiptap p::after,
+.paper.show-formatting-marks .tiptap h1::after,
+.paper.show-formatting-marks .tiptap h2::after,
+.paper.show-formatting-marks .tiptap h3::after {
+  content: '\00A0¶';
+  display: inline-block;
+  width: 0;
+  overflow: visible;
+  white-space: pre;
+  color: var(--color-primary);
+  opacity: 0.5;
+  font-family: var(--font-sans);
+  font-weight: 400;
+  user-select: none;
+  pointer-events: none;
+}
+
+.paper.show-formatting-marks .tiptap li > p::after {
+  content: '\00A0¶';
+}
+
+/* Empty block fix: when a paragraph/heading/li-paragraph contains only the
+   ProseMirror trailing-break BR, the inline ::after would otherwise render
+   on a second line below the BR, doubling the block's height and shifting
+   the page-break calculation. Pull ::after out of flow via absolute positioning. */
+.paper.show-formatting-marks .tiptap p:has(> br.ProseMirror-trailingBreak:only-child),
+.paper.show-formatting-marks .tiptap h1:has(> br.ProseMirror-trailingBreak:only-child),
+.paper.show-formatting-marks .tiptap h2:has(> br.ProseMirror-trailingBreak:only-child),
+.paper.show-formatting-marks .tiptap h3:has(> br.ProseMirror-trailingBreak:only-child),
+.paper.show-formatting-marks .tiptap li > p:has(> br.ProseMirror-trailingBreak:only-child) {
+  position: relative;
+}
+
+.paper.show-formatting-marks .tiptap p:has(> br.ProseMirror-trailingBreak:only-child)::after,
+.paper.show-formatting-marks .tiptap h1:has(> br.ProseMirror-trailingBreak:only-child)::after,
+.paper.show-formatting-marks .tiptap h2:has(> br.ProseMirror-trailingBreak:only-child)::after,
+.paper.show-formatting-marks .tiptap h3:has(> br.ProseMirror-trailingBreak:only-child)::after,
+.paper.show-formatting-marks .tiptap li > p:has(> br.ProseMirror-trailingBreak:only-child)::after {
+  position: absolute;
+  top: 0;
+  left: 0;
+  margin-left: 0;
+}
+
+.paper.show-formatting-marks .tiptap .pm-space-mark {
+  position: relative;
+}
+
+.paper.show-formatting-marks .tiptap .pm-space-mark::before {
+  content: '·';
+  position: absolute;
+  inset: 0;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  color: var(--color-primary);
+  opacity: 0.55;
+  font-weight: 700;
+  pointer-events: none;
+  user-select: none;
+}
+
 /* Print styles: proper A4 pagination */
 @media print {
   .editor {
@@ -185,4 +252,13 @@
     size: A4;
     margin: 25.4mm;
   }
+
+  .paper.show-formatting-marks .tiptap p::after,
+  .paper.show-formatting-marks .tiptap h1::after,
+  .paper.show-formatting-marks .tiptap h2::after,
+  .paper.show-formatting-marks .tiptap h3::after,
+  .paper.show-formatting-marks .tiptap li > p::after,
+  .paper.show-formatting-marks .tiptap .pm-space-mark::before {
+    content: none;
+  }
 }