diff --git a/src/sidebar/components/Annotation/AnnotationEditor.tsx b/src/sidebar/components/Annotation/AnnotationEditor.tsx
index 30d76ec2593..570a96fa345 100644
--- a/src/sidebar/components/Annotation/AnnotationEditor.tsx
+++ b/src/sidebar/components/Annotation/AnnotationEditor.tsx
@@ -167,6 +167,12 @@ function AnnotationEditor({
 
   const textStyle = applyTheme(['annotationFontFamily'], settings);
 
+  const atMentionsEnabled = store.isFeatureEnabled('at_mentions');
+  const usersMatchingTerm = useCallback(
+    (term: string) => store.usersWhoAnnotated(term),
+    [store],
+  );
+
   return (
     /* eslint-disable-next-line jsx-a11y/no-static-element-interactions */
     <div
@@ -179,7 +185,8 @@ function AnnotationEditor({
         label={isReplyAnno ? 'Enter reply' : 'Enter comment'}
         text={text}
         onEditText={onEditText}
-        atMentionsEnabled={store.isFeatureEnabled('at_mentions')}
+        atMentionsEnabled={atMentionsEnabled}
+        usersMatchingTerm={usersMatchingTerm}
       />
       <TagEditor
         onAddTag={onAddTag}
diff --git a/src/sidebar/components/MarkdownEditor.tsx b/src/sidebar/components/MarkdownEditor.tsx
index d3b6502c317..dfb97257f4c 100644
--- a/src/sidebar/components/MarkdownEditor.tsx
+++ b/src/sidebar/components/MarkdownEditor.tsx
@@ -181,20 +181,36 @@ function ToolbarButton({
   );
 }
 
+type UserItem = {
+  user: string;
+  displayName: string | null;
+};
+
 type TextAreaProps = {
   classes?: string;
   containerRef?: Ref<HTMLTextAreaElement>;
   atMentionsEnabled: boolean;
+  usersMatchingTerm: (mention: string) => UserItem[];
 };
 
 function TextArea({
   classes,
   containerRef,
   atMentionsEnabled,
+  usersMatchingTerm,
   ...restProps
 }: TextAreaProps & JSX.TextareaHTMLAttributes<HTMLTextAreaElement>) {
   const [popoverOpen, setPopoverOpen] = useState(false);
+  const [activeMention, setActiveMention] = useState<string>();
   const textareaRef = useSyncedRef(containerRef);
+  const [highlightedSuggestion, setHighlightedSuggestion] = useState(0);
+  const suggestions = useMemo(() => {
+    if (!atMentionsEnabled || activeMention === undefined) {
+      return [];
+    }
+
+    return usersMatchingTerm(activeMention);
+  }, [activeMention, atMentionsEnabled, usersMatchingTerm]);
 
   useEffect(() => {
     if (!atMentionsEnabled) {
@@ -212,17 +228,46 @@ function TextArea({
       if (e.key === 'Escape') {
         return;
       }
-      setPopoverOpen(
-        termBeforePosition(textarea.value, textarea.selectionStart).startsWith(
-          '@',
-        ),
+
+      const termBeforeCaret = termBeforePosition(
+        textarea.value,
+        textarea.selectionStart,
       );
+      const isAtMention = termBeforeCaret.startsWith('@');
+
+      setPopoverOpen(isAtMention);
+      setActiveMention(isAtMention ? termBeforeCaret.substring(1) : undefined);
+    });
+
+    listenerCollection.add(textarea, 'keydown', e => {
+      if (
+        !popoverOpen ||
+        suggestions.length === 0 ||
+        !['ArrowDown', 'ArrowUp', 'Enter'].includes(e.key)
+      ) {
+        return;
+      }
+
+      // When vertical arrows or Enter are pressed while the popover is open
+      // with suggestions, highlight or pick the right suggestion.
+      e.preventDefault();
+      e.stopPropagation();
+
+      if (e.key === 'ArrowDown') {
+        setHighlightedSuggestion(prev =>
+          Math.min(prev + 1, suggestions.length),
+        );
+      } else if (e.key === 'ArrowUp') {
+        setHighlightedSuggestion(prev => Math.max(prev - 1, 0));
+      } else if (e.key === 'Enter') {
+        // TODO "Print" suggestion in textarea
+      }
     });
 
     return () => {
       listenerCollection.removeAll();
     };
-  }, [atMentionsEnabled, popoverOpen, textareaRef]);
+  }, [atMentionsEnabled, popoverOpen, suggestions.length, textareaRef]);
 
   return (
     <div className="relative">
@@ -241,9 +286,30 @@ function TextArea({
           open={popoverOpen}
           onClose={() => setPopoverOpen(false)}
           anchorElementRef={textareaRef}
-          classes="p-2"
+          classes="p-1"
         >
-          Suggestions
+          <ul className="flex-col gap-y-0.5">
+            {suggestions.map((s, index) => (
+              <li
+                key={s.user}
+                className={classnames(
+                  'flex justify-between items-center',
+                  'rounded p-2 hover:bg-grey-2',
+                  {
+                    'bg-grey-2': highlightedSuggestion === index,
+                  },
+                )}
+              >
+                <span className="truncate">{s.user}</span>
+                <span className="text-color-text-light">{s.displayName}</span>
+              </li>
+            ))}
+            {suggestions.length === 0 && (
+              <li className="italic p-2">
+                No matches. You can still write the username
+              </li>
+            )}
+          </ul>
         </Popover>
       )}
     </div>
@@ -391,6 +457,13 @@ export type MarkdownEditorProps = {
   text: string;
 
   onEditText?: (text: string) => void;
+
+  /**
+   * A function that returns a list of users which username or display name
+   * match provided term.
+   * This is used only if `atMentionsEnabled` is `true`.
+   */
+  usersMatchingTerm: (term: string) => UserItem[];
 };
 
 /**
@@ -402,6 +475,7 @@ export default function MarkdownEditor({
   onEditText = () => {},
   text,
   textStyle = {},
+  usersMatchingTerm,
 }: MarkdownEditorProps) {
   // Whether the preview mode is currently active.
   const [preview, setPreview] = useState(false);
@@ -472,6 +546,7 @@ export default function MarkdownEditor({
           value={text}
           style={textStyle}
           atMentionsEnabled={atMentionsEnabled}
+          usersMatchingTerm={usersMatchingTerm}
         />
       )}
     </div>
diff --git a/src/sidebar/store/modules/annotations.ts b/src/sidebar/store/modules/annotations.ts
index 1ad0dad2b7b..9b635c8fd72 100644
--- a/src/sidebar/store/modules/annotations.ts
+++ b/src/sidebar/store/modules/annotations.ts
@@ -8,6 +8,7 @@ import { createSelector } from 'reselect';
 import { hasOwn } from '../../../shared/has-own';
 import type { Annotation, SavedAnnotation } from '../../../types/api';
 import type { HighlightCluster } from '../../../types/shared';
+import { username } from '../../helpers/account-id';
 import * as metadata from '../../helpers/annotation-metadata';
 import { isHighlight, isSaved } from '../../helpers/annotation-metadata';
 import { countIf, toTrueMap, trueKeys } from '../../util/collections';
@@ -34,6 +35,11 @@ type AnnotationStub = {
   $tag?: string;
 };
 
+export type UserItem = {
+  user: string;
+  displayName: string | null;
+};
+
 const initialState = {
   annotations: [],
   highlighted: {},
@@ -567,6 +573,40 @@ const savedAnnotations = createSelector(
   annotations => annotations.filter(ann => isSaved(ann)) as SavedAnnotation[],
 );
 
+/**
+ * Return a list of unique users who authored any annotation, ordered by username.
+ * Optionally filter and slice the list
+ */
+function usersWhoAnnotated({ annotations }: State, term?: string, max = 10) {
+  const usersMap = new Map<
+    string,
+    { user: string; displayName: string | null }
+  >();
+  annotations.forEach(anno => {
+    const user = username(anno.user);
+    const displayName = anno.user_info?.display_name ?? null;
+
+    if (
+      // Keep a unique list of users
+      !usersMap.has(user) &&
+      // Match all users if the term is empty
+      (!term || `${user} ${displayName ?? ''}`.match(term))
+    ) {
+      usersMap.set(user, { user, displayName });
+    }
+  });
+
+  // Sort users by username and pick the top 10
+  return [...usersMap.values()]
+    .sort((a, b) => {
+      const lowerAUsername = a.user.toLowerCase();
+      const lowerBUsername = b.user.toLowerCase();
+
+      return lowerAUsername.localeCompare(lowerBUsername);
+    })
+    .slice(0, max);
+}
+
 export const annotationsModule = createStoreModule(initialState, {
   namespace: 'annotations',
   reducers,
@@ -597,5 +637,6 @@ export const annotationsModule = createStoreModule(initialState, {
     noteCount,
     orphanCount,
     savedAnnotations,
+    usersWhoAnnotated,
   },
 });