diff --git a/packages/backend/src/server/api/stream/Connection.ts b/packages/backend/src/server/api/stream/Connection.ts
index 9ca209f08b..36e5769216 100644
--- a/packages/backend/src/server/api/stream/Connection.ts
+++ b/packages/backend/src/server/api/stream/Connection.ts
@@ -23,6 +23,7 @@ import type { EventEmitter } from 'events';
 import type Channel from './channel.js';
 
 const MAX_CHANNELS_PER_CONNECTION = 32;
+const MAX_SUBSCRIPTIONS_PER_CONNECTION = 256;
 
 /**
  * Main stream connection
@@ -34,7 +35,7 @@ export default class Connection {
 	private wsConnection: WebSocket.WebSocket;
 	public subscriber: StreamEventEmitter;
 	private channels: Channel[] = [];
-	private subscribingNotes: Partial<Record<string, number>> = {};
+	private subscribingNotes = new Map<string, number>();
 	private cachedNotes: Packed<'Note'>[] = [];
 	public userProfile: MiUserProfile | null = null;
 	public following: Record<string, Pick<MiFollowing, 'withReplies'> | undefined> = {};
@@ -200,9 +201,21 @@ export default class Connection {
 		if (!isJsonObject(payload)) return;
 		if (!payload.id || typeof payload.id !== 'string') return;
 
-		const current = this.subscribingNotes[payload.id] ?? 0;
+		const current = this.subscribingNotes.get(payload.id) ?? 0;
+
+		// Limit the number of distinct notes that can be subscribed to.
+		// If current is-zero, then this is a new note and we need to check the limit
+		if (current === 0 && this.subscribingNotes.size >= MAX_SUBSCRIPTIONS_PER_CONNECTION) {
+			// Map maintains insertion order, so first key is always the oldest
+			// eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+			const oldestKey = this.subscribingNotes.keys().next().value!;
+
+			this.subscribingNotes.delete(oldestKey);
+			this.subscriber.off(`noteStream:${oldestKey}`, this.onNoteStreamMessage);
+		}
+
 		const updated = current + 1;
-		this.subscribingNotes[payload.id] = updated;
+		this.subscribingNotes.set(payload.id, updated);
 
 		if (updated === 1) {
 			this.subscriber.on(`noteStream:${payload.id}`, this.onNoteStreamMessage);
@@ -217,12 +230,12 @@ export default class Connection {
 		if (!isJsonObject(payload)) return;
 		if (!payload.id || typeof payload.id !== 'string') return;
 
-		const current = this.subscribingNotes[payload.id];
+		const current = this.subscribingNotes.get(payload.id);
 		if (current == null) return;
 		const updated = current - 1;
-		this.subscribingNotes[payload.id] = updated;
+		this.subscribingNotes.set(payload.id, updated);
 		if (updated <= 0) {
-			delete this.subscribingNotes[payload.id];
+			this.subscribingNotes.delete(payload.id);
 			this.subscriber.off(`noteStream:${payload.id}`, this.onNoteStreamMessage);
 		}
 	}