diff --git a/src/apps/notes/index.tsx b/src/apps/notes/index.tsx
index a7f2155..a88872d 100644
--- a/src/apps/notes/index.tsx
+++ b/src/apps/notes/index.tsx
@@ -7,6 +7,7 @@ import { AppBody, AppLayout, AppToolbar, EmptyState } from '@/components/os/AppC
 import { AuthorLine } from '@/components/nostr/AuthorLine';
 import { NoteContent } from '@/components/nostr/NoteContent';
 import { NoteCard } from '@/components/nostr/NoteCard';
+import { ReactionButton } from '@/components/nostr/ReactionButton';
 import { Composer } from '@/apps/feed/Composer';
 import { DraftNote } from './Draft';
 import { Button } from '@/components/ui/button';
@@ -121,7 +122,10 @@ export default function NotesApp({ params, setTitle, setParams }: AppProps) {
           <div className="mt-3">
             <NoteContent content={event.content} className="text-base" />
           </div>
-          <p className="mt-3 text-xs text-muted-foreground">{absoluteTime(event.created_at)}</p>
+          <div className="mt-3 flex items-center gap-3">
+            <p className="text-xs text-muted-foreground">{absoluteTime(event.created_at)}</p>
+            <ReactionButton target={event} className="h-6" />
+          </div>
         </div>
 
         {user && (
diff --git a/src/components/nostr/NoteCard.tsx b/src/components/nostr/NoteCard.tsx
index cfd9146..59e746b 100644
--- a/src/components/nostr/NoteCard.tsx
+++ b/src/components/nostr/NoteCard.tsx
@@ -4,6 +4,7 @@ import { nip19 } from 'nostr-tools';
 import { AuthorLine } from './AuthorLine';
 import { NoteContent } from './NoteContent';
 import { BookmarkButton } from './BookmarkButton';
+import { ReactionButton } from './ReactionButton';
 import { Button } from '@/components/ui/button';
 import { useWindowManager } from '@/os/useWindowManager';
 import { useToast } from '@/hooks/useToast';
@@ -70,6 +71,7 @@ export function NoteCard({ event, compact, className }: NoteCardProps) {
               <Repeat2 className="size-3.5" aria-hidden />
               Copy link
             </Button>
+            <ReactionButton target={event} />
             <BookmarkButton target={{ type: 'e', value: event.id }} />
           </div>
         )}
diff --git a/src/components/nostr/ReactionButton.tsx b/src/components/nostr/ReactionButton.tsx
new file mode 100644
index 0000000..193f762
--- /dev/null
+++ b/src/components/nostr/ReactionButton.tsx
@@ -0,0 +1,73 @@
+import { useState } from 'react';
+import { Heart, Loader2 } from 'lucide-react';
+import type { NostrEvent } from '@nostrify/nostrify';
+import { Button } from '@/components/ui/button';
+import AuthDialog from '@/components/auth/AuthDialog';
+import { useCurrentUser } from '@/hooks/useCurrentUser';
+import { useToast } from '@/hooks/useToast';
+import { useReactions, summarizeReactions, useToggleReaction } from '@/hooks/useReactions';
+import { cn } from '@/lib/utils';
+
+/** Likes `target` (a note or reply) via NIP-25 reactions, from the feed or a thread. */
+export function ReactionButton({ target, className }: { target: NostrEvent; className?: string }) {
+  const { user } = useCurrentUser();
+  const { toast } = useToast();
+  const [authOpen, setAuthOpen] = useState(false);
+  const reactions = useReactions(target.id);
+  const toggle = useToggleReaction();
+
+  const { count, byAuthor } = summarizeReactions(reactions.data);
+  const own = user ? byAuthor.get(user.pubkey) : undefined;
+  const reacted = Boolean(own && own.content !== '-');
+  // Kind 7 isn't replaceable, so the viewer may have more than one reaction
+  // event on this note; un-reacting needs to clear all of them, not just the
+  // one `summarizeReactions` picked as "latest".
+  const ownReactions = user ? (reactions.data ?? []).filter((event) => event.pubkey === user.pubkey) : [];
+
+  const handleClick = () => {
+    if (!user) {
+      setAuthOpen(true);
+      return;
+    }
+    toggle.mutate(
+      { target, ownReactions: reacted ? ownReactions : undefined },
+      {
+        onError: (error) => {
+          toast({
+            title: reacted ? 'Could not remove like' : 'Could not like',
+            description: error instanceof Error ? error.message : 'No relay accepted the update.',
+            variant: 'destructive',
+          });
+        },
+      },
+    );
+  };
+
+  const label = reacted
+    ? `Remove your like${count > 0 ? ` — ${count} ${count === 1 ? 'like' : 'likes'}` : ''}`
+    : `Like${count > 0 ? ` — ${count} ${count === 1 ? 'like' : 'likes'}` : ''}`;
+
+  return (
+    <>
+      <Button
+        type="button"
+        variant="ghost"
+        size="sm"
+        className={cn('h-7 gap-1.5 px-2 text-xs text-muted-foreground', reacted && 'text-rose-500', className)}
+        onClick={handleClick}
+        disabled={toggle.isPending}
+        aria-pressed={reacted}
+        aria-label={label}
+      >
+        {toggle.isPending ? (
+          <Loader2 className="size-3.5 animate-spin" aria-hidden />
+        ) : (
+          <Heart className={cn('size-3.5', reacted && 'fill-current')} aria-hidden />
+        )}
+        <span aria-hidden>{count > 0 ? count : ''}</span>
+      </Button>
+
+      <AuthDialog isOpen={authOpen} onClose={() => setAuthOpen(false)} />
+    </>
+  );
+}
diff --git a/src/hooks/useReactions.test.ts b/src/hooks/useReactions.test.ts
new file mode 100644
index 0000000..c084ea2
--- /dev/null
+++ b/src/hooks/useReactions.test.ts
@@ -0,0 +1,31 @@
+import { describe, expect, it } from 'vitest';
+import type { NostrEvent } from '@nostrify/nostrify';
+import { summarizeReactions } from './useReactions';
+
+function reaction(pubkey: string, content: string, createdAt: number): NostrEvent {
+  return { id: `${pubkey}-${createdAt}`, pubkey, created_at: createdAt, kind: 7, tags: [], content, sig: '' };
+}
+
+describe('summarizeReactions', () => {
+  it('counts each author once', () => {
+    const events = [reaction('alice', '+', 1), reaction('bob', '+', 2)];
+    expect(summarizeReactions(events).count).toBe(2);
+  });
+
+  it('keeps only the latest reaction per author', () => {
+    const events = [reaction('alice', '+', 1), reaction('alice', '-', 2)];
+    const summary = summarizeReactions(events);
+    expect(summary.count).toBe(0);
+    expect(summary.byAuthor.get('alice')?.content).toBe('-');
+  });
+
+  it('excludes downvotes from the count', () => {
+    const events = [reaction('alice', '+', 1), reaction('bob', '-', 1)];
+    expect(summarizeReactions(events).count).toBe(1);
+  });
+
+  it('returns zero for no reactions', () => {
+    expect(summarizeReactions(undefined).count).toBe(0);
+    expect(summarizeReactions([]).byAuthor.size).toBe(0);
+  });
+});
diff --git a/src/hooks/useReactions.ts b/src/hooks/useReactions.ts
new file mode 100644
index 0000000..2f0ebed
--- /dev/null
+++ b/src/hooks/useReactions.ts
@@ -0,0 +1,152 @@
+import { useNostr } from '@nostrify/react';
+import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
+import type { NostrEvent } from '@nostrify/nostrify';
+import { useCurrentUser } from './useCurrentUser';
+import { useNostrPublish } from './useNostrPublish';
+
+/** NIP-25 reactions. Content `-` is a downvote; anything else (commonly `+`) is a like. */
+const REACTION_KIND = 7;
+const DELETION_KIND = 5;
+
+function reactionsQueryKey(eventId: string) {
+  return ['nostr', 'reactions', eventId] as const;
+}
+
+/** All kind-7 reactions to `eventId`, newest first. */
+export function useReactions(eventId: string | undefined) {
+  const { nostr } = useNostr();
+
+  return useQuery<NostrEvent[]>({
+    queryKey: reactionsQueryKey(eventId ?? ''),
+    enabled: Boolean(eventId),
+    queryFn: async ({ signal }) => {
+      const events = await nostr.query(
+        [{ kinds: [REACTION_KIND], '#e': [eventId!], limit: 500 }],
+        { signal: AbortSignal.any([signal, AbortSignal.timeout(6000)]) },
+      );
+      return events.sort((a, b) => b.created_at - a.created_at);
+    },
+    staleTime: 30_000,
+  });
+}
+
+/** Only the most recent reaction per author — an author can change their mind. */
+function latestPerAuthor(events: NostrEvent[]): Map<string, NostrEvent> {
+  const byAuthor = new Map<string, NostrEvent>();
+  for (const event of events) {
+    const existing = byAuthor.get(event.pubkey);
+    if (!existing || event.created_at > existing.created_at) {
+      byAuthor.set(event.pubkey, event);
+    }
+  }
+  return byAuthor;
+}
+
+export interface ReactionSummary {
+  /** Distinct authors whose latest reaction is a like (i.e. not a `-` downvote). */
+  count: number;
+  byAuthor: Map<string, NostrEvent>;
+}
+
+export function summarizeReactions(events: NostrEvent[] | undefined): ReactionSummary {
+  const byAuthor = latestPerAuthor(events ?? []);
+  let count = 0;
+  for (const event of byAuthor.values()) {
+    if (event.content !== '-') count++;
+  }
+  return { count, byAuthor };
+}
+
+interface ToggleReactionInput {
+  /** The note or reply being reacted to. */
+  target: NostrEvent;
+  /**
+   * All of the signed-in user's own reaction events on `target`, if any —
+   * pass to un-react. Kind 7 is a regular (non-replaceable) event, so a user
+   * can end up with more than one over time (races, retries, multiple
+   * devices); every one of them needs deleting, not just the newest.
+   */
+  ownReactions?: NostrEvent[];
+}
+
+/**
+ * Likes or un-likes a note. Un-reacting publishes a single NIP-09 deletion
+ * covering *all* of the viewer's own reaction events on the target, rather
+ * than just the most recently seen one — most relays and clients honor
+ * deletions, whereas a `-` reaction would just add another, conflicting
+ * event without necessarily retracting the others. Deleting only the latest
+ * would leave any older `+` in place to resurface as "the" reaction (and
+ * re-inflate the count) once relays stop returning the deleted one.
+ */
+export function useToggleReaction() {
+  const { user } = useCurrentUser();
+  const publish = useNostrPublish();
+  const queryClient = useQueryClient();
+
+  return useMutation({
+    mutationFn: async ({ target, ownReactions }: ToggleReactionInput) => {
+      if (!user) throw new Error('Sign in to react');
+      if (ownReactions && ownReactions.length > 0) {
+        return publish.mutateAsync({
+          kind: DELETION_KIND,
+          content: '',
+          tags: [...ownReactions.map((event): [string, string] => ['e', event.id]), ['k', String(REACTION_KIND)]],
+        });
+      }
+      return publish.mutateAsync({
+        kind: REACTION_KIND,
+        content: '+',
+        tags: [
+          ['e', target.id],
+          ['p', target.pubkey],
+          ['k', target.kind.toString()],
+        ],
+      });
+    },
+    onMutate: async ({ target, ownReactions }) => {
+      if (!user) return undefined;
+      const key = reactionsQueryKey(target.id);
+      await queryClient.cancelQueries({ queryKey: key });
+      const previous = queryClient.getQueryData<NostrEvent[]>(key);
+
+      queryClient.setQueryData<NostrEvent[]>(key, (old = []) => {
+        const withoutMine = old.filter((event) => event.pubkey !== user.pubkey);
+        if (ownReactions && ownReactions.length > 0) return withoutMine;
+        const optimistic: NostrEvent = {
+          id: `optimistic:${target.id}:${user.pubkey}`,
+          pubkey: user.pubkey,
+          created_at: Math.floor(Date.now() / 1000),
+          kind: REACTION_KIND,
+          content: '+',
+          tags: [['e', target.id], ['p', target.pubkey]],
+          sig: '',
+        };
+        return [optimistic, ...withoutMine];
+      });
+
+      return { previous, key };
+    },
+    onError: (_error, _variables, context) => {
+      if (context) {
+        queryClient.setQueryData(context.key, context.previous);
+      }
+    },
+    // Deliberately not `invalidateQueries` here: right after a successful
+    // publish, relays are eventually consistent, so an immediate re-query
+    // commonly hits one that hasn't indexed the new event yet — the stale
+    // result would silently overwrite the correct state a moment later
+    // (confirmed live: a like reverted to "unliked" ~1s after publishing).
+    // Swapping in the mutation's own known-correct result is also required,
+    // not just safer: `onMutate`'s optimistic entry uses a fake
+    // `optimistic:...` id, and a like followed immediately by an unlike needs
+    // the *real* signed event id to build a deletion relays will honor —
+    // without this, that later delete would target an id that never existed.
+    onSuccess: (publishedEvent, { target, ownReactions }) => {
+      if (!user) return;
+      queryClient.setQueryData<NostrEvent[]>(reactionsQueryKey(target.id), (old = []) => {
+        const withoutMine = old.filter((event) => event.pubkey !== user.pubkey);
+        return ownReactions && ownReactions.length > 0 ? withoutMine : [publishedEvent, ...withoutMine];
+      });
+    },
+  });
+}