diff --git a/app/javascript/mastodon/features/collections/editor/details.tsx b/app/javascript/mastodon/features/collections/editor/details.tsx
index 875d09c9ebb..f59bd4de51b 100644
--- a/app/javascript/mastodon/features/collections/editor/details.tsx
+++ b/app/javascript/mastodon/features/collections/editor/details.tsx
@@ -6,6 +6,7 @@ import { useHistory } from 'react-router-dom';
 
 import { isFulfilled } from '@reduxjs/toolkit';
 
+import { inputToHashtag } from '@/mastodon/utils/hashtags';
 import type {
   ApiCreateCollectionPayload,
   ApiUpdateCollectionPayload,
@@ -64,7 +65,7 @@ export const CollectionDetails: React.FC = () => {
       dispatch(
         updateCollectionEditorField({
           field: 'topic',
-          value: event.target.value,
+          value: inputToHashtag(event.target.value),
         }),
       );
     },
@@ -219,6 +220,9 @@ export const CollectionDetails: React.FC = () => {
           }
           value={topic}
           onChange={handleTopicChange}
+          autoCapitalize='off'
+          autoCorrect='off'
+          spellCheck='false'
           maxLength={40}
         />
 
diff --git a/app/javascript/mastodon/reducers/slices/collections.ts b/app/javascript/mastodon/reducers/slices/collections.ts
index dc20b987323..a534a134404 100644
--- a/app/javascript/mastodon/reducers/slices/collections.ts
+++ b/app/javascript/mastodon/reducers/slices/collections.ts
@@ -37,30 +37,30 @@ interface CollectionState {
       status: QueryStatus;
     }
   >;
-  editor: {
-    id: string | undefined;
-    name: string;
-    description: string;
-    topic: string;
-    language: string | null;
-    discoverable: boolean;
-    sensitive: boolean;
-    accountIds: string[];
-  };
+  editor: EditorState;
 }
 
-type EditorField = CollectionState['editor'];
+interface EditorState {
+  id: string | null;
+  name: string;
+  description: string;
+  topic: string;
+  language: string | null;
+  discoverable: boolean;
+  sensitive: boolean;
+  accountIds: string[];
+}
 
-interface UpdateEditorFieldPayload<K extends keyof EditorField> {
+interface UpdateEditorFieldPayload<K extends keyof EditorState> {
   field: K;
-  value: EditorField[K];
+  value: EditorState[K];
 }
 
 const initialState: CollectionState = {
   collections: {},
   accountCollections: {},
   editor: {
-    id: undefined,
+    id: null,
     name: '',
     description: '',
     topic: '',
@@ -79,7 +79,7 @@ const collectionSlice = createSlice({
       const collection = action.payload;
 
       state.editor = {
-        id: collection?.id,
+        id: collection?.id ?? null,
         name: collection?.name ?? '',
         description: collection?.description ?? '',
         topic: collection?.tag?.name ?? '',
@@ -92,7 +92,7 @@ const collectionSlice = createSlice({
     reset(state) {
       state.editor = initialState.editor;
     },
-    updateEditorField<K extends keyof EditorField>(
+    updateEditorField<K extends keyof EditorState>(
       state: CollectionState,
       action: PayloadAction<UpdateEditorFieldPayload<K>>,
     ) {
diff --git a/app/javascript/mastodon/utils/hashtags.test.ts b/app/javascript/mastodon/utils/hashtags.test.ts
new file mode 100644
index 00000000000..05b79b1d52c
--- /dev/null
+++ b/app/javascript/mastodon/utils/hashtags.test.ts
@@ -0,0 +1,28 @@
+import { inputToHashtag } from './hashtags';
+
+describe('inputToHashtag', () => {
+  test.concurrent.each([
+    ['', ''],
+    // Prepend or keep hashtag
+    ['mastodon', '#mastodon'],
+    ['#mastodon', '#mastodon'],
+    // Preserve trailing whitespace
+    ['mastodon  ', '#mastodon  '],
+    ['   ', '#   '],
+    // Collapse whitespace & capitalise first character
+    ['cats of mastodon', '#catsOfMastodon'],
+    ['x y z', '#xYZ'],
+    ['   mastodon', '#mastodon'],
+    // Preserve initial casing
+    ['Log in', '#LogIn'],
+    ['#NaturePhotography', '#NaturePhotography'],
+    // Normalise hash symbol variant
+    ['＃nature', '#nature'],
+    ['＃Nature Photography', '#NaturePhotography'],
+    // Allow special characters
+    ['hello-world', '#hello-world'],
+    ['hello,world', '#hello,world'],
+  ])('for input "%s", return "%s"', (input, expected) => {
+    expect(inputToHashtag(input)).toBe(expected);
+  });
+});
diff --git a/app/javascript/mastodon/utils/hashtags.ts b/app/javascript/mastodon/utils/hashtags.ts
index 0c5505c6c9a..d14efe5db33 100644
--- a/app/javascript/mastodon/utils/hashtags.ts
+++ b/app/javascript/mastodon/utils/hashtags.ts
@@ -27,3 +27,35 @@ const buildHashtagRegex = () => {
 export const HASHTAG_PATTERN_REGEX = buildHashtagPatternRegex();
 
 export const HASHTAG_REGEX = buildHashtagRegex();
+
+/**
+ * Formats an input string as a hashtag:
+ * - Prepends `#` unless present
+ * - Strips spaces (except at the end, to allow typing it)
+ * - Capitalises first character after stripped space
+ */
+export const inputToHashtag = (input: string): string => {
+  if (!input) {
+    return '';
+  }
+
+  const trailingSpace = /\s+$/.exec(input)?.[0] ?? '';
+  const trimmedInput = input.trimEnd();
+
+  const withoutHash =
+    trimmedInput.startsWith('#') || trimmedInput.startsWith('＃')
+      ? trimmedInput.slice(1)
+      : trimmedInput;
+
+  // Split by space, filter empty strings, and capitalise the start of each word but the first
+  const words = withoutHash
+    .split(/\s+/)
+    .filter((word) => word.length > 0)
+    .map((word, index) =>
+      index === 0
+        ? word
+        : word.charAt(0).toUpperCase() + word.slice(1).toLowerCase(),
+    );
+
+  return `#${words.join('')}${trailingSpace}`;
+};