diff --git a/.config/docker_example.yml b/.config/docker_example.yml
index 3f8e5734ce..17b835480f 100644
--- a/.config/docker_example.yml
+++ b/.config/docker_example.yml
@@ -219,3 +219,7 @@ signToActivityPubGet: true
 
 # Upload or download file size limits (bytes)
 #maxFileSize: 262144000
+
+defaultTag:
+  tag: null
+  append: true
diff --git a/.config/example.yml b/.config/example.yml
index 60a6a0aa71..9a6673064d 100644
--- a/.config/example.yml
+++ b/.config/example.yml
@@ -321,3 +321,7 @@ signToActivityPubGet: true
 
 # PID File of master process
 #pidFile: /tmp/misskey.pid
+
+defaultTag:
+  tag: null
+  append: true
diff --git a/packages/backend/src/config.ts b/packages/backend/src/config.ts
index 42f1033b9d..6ad6f8c848 100644
--- a/packages/backend/src/config.ts
+++ b/packages/backend/src/config.ts
@@ -60,6 +60,10 @@ type Source = {
 	};
 	sentryForBackend?: { options: Partial<Sentry.NodeOptions>; enableNodeProfiling: boolean; };
 	sentryForFrontend?: { options: Partial<Sentry.NodeOptions> };
+	defaultTag: {
+		tag: string;
+		append: boolean;
+	};
 
 	publishTarballInsteadOfProvideRepositoryUrl?: boolean;
 
@@ -132,6 +136,10 @@ export type Config = {
 		index: string;
 		scope?: 'local' | 'global' | string[];
 	} | undefined;
+	defaultTag: {
+		tag: string;
+		append: boolean;
+	};
 	proxy: string | undefined;
 	proxySmtp: string | undefined;
 	proxyBypassHosts: string[] | undefined;
@@ -293,6 +301,7 @@ export function loadConfig(): Config {
 		perUserNotificationsMaxCount: config.perUserNotificationsMaxCount ?? 500,
 		deactivateAntennaThreshold: config.deactivateAntennaThreshold ?? (1000 * 60 * 60 * 24 * 7),
 		pidFile: config.pidFile,
+		defaultTag: config.defaultTag,
 	};
 }
 
diff --git a/packages/backend/src/core/NoteCreateService.ts b/packages/backend/src/core/NoteCreateService.ts
index 3647fa7231..e95b54132b 100644
--- a/packages/backend/src/core/NoteCreateService.ts
+++ b/packages/backend/src/core/NoteCreateService.ts
@@ -330,6 +330,16 @@ export class NoteCreateService implements OnApplicationShutdown {
 			data.localOnly = true;
 		}
 
+		// デフォルトハッシュタグを本文末尾に書き足す
+		if (this.config.defaultTag?.append && ['public', 'home'].includes(data.visibility)) {
+			if (this.config.defaultTag?.tag != null) {
+				const tag = `#${this.config.defaultTag?.tag}`;
+				if (String(data.text).match(tag)) {
+					data.text = `${data.text}\n\n${tag}`;
+				}
+			}
+		}
+
 		if (data.text) {
 			if (data.text.length > DB_MAX_NOTE_TEXT_LENGTH) {
 				data.text = data.text.slice(0, DB_MAX_NOTE_TEXT_LENGTH);
@@ -944,6 +954,18 @@ export class NoteCreateService implements OnApplicationShutdown {
 				}
 			}
 
+			// デフォルトハッシュタグを含む投稿は、リモートであってもローカルタイムラインに含める
+			if (this.config.defaultTag?.tag != null) {
+				const noteTags = note.tags ? note.tags.map((t: string) => t.toLowerCase()) : [];
+				if (note.visibility === 'public' && noteTags.includes(normalizeForSearch(this.config.defaultTag?.tag))) {
+					this.fanoutTimelineService.push('localTimelineWithReplies', note.id, 300, r);
+					this.fanoutTimelineService.push('localTimeline', note.id, 1000, r);
+					if (note.fileIds.length > 0) {
+						this.fanoutTimelineService.push('localTimelineWithFiles', note.id, 500, r);
+					}
+				}
+			}
+
 			// 自分自身以外への返信
 			if (isReply(note)) {
 				this.fanoutTimelineService.push(`userTimelineWithReplies:${user.id}`, note.id, note.userHost == null ? this.meta.perLocalUserUserTimelineCacheMax : this.meta.perRemoteUserUserTimelineCacheMax, r);
diff --git a/packages/backend/src/server/api/endpoints/notes/hybrid-timeline.ts b/packages/backend/src/server/api/endpoints/notes/hybrid-timeline.ts
index aed9065bf9..0582a84493 100644
--- a/packages/backend/src/server/api/endpoints/notes/hybrid-timeline.ts
+++ b/packages/backend/src/server/api/endpoints/notes/hybrid-timeline.ts
@@ -19,6 +19,8 @@ import { UserFollowingService } from '@/core/UserFollowingService.js';
 import { MiLocalUser } from '@/models/User.js';
 import { FanoutTimelineEndpointService } from '@/core/FanoutTimelineEndpointService.js';
 import { ApiError } from '../../error.js';
+import { normalizeForSearch } from '@/misc/normalize-for-search.js';
+import { loadConfig } from '@/config.js';
 
 export const meta = {
 	tags: ['notes'],
@@ -207,10 +209,16 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				if (followees.length > 0) {
 					const meOrFolloweeIds = [me.id, ...followees.map(f => f.followeeId)];
 					qb.where('note.userId IN (:...meOrFolloweeIds)', { meOrFolloweeIds: meOrFolloweeIds });
-					qb.orWhere('(note.visibility = \'public\') AND (note.userHost IS NULL)');
 				} else {
 					qb.where('note.userId = :meId', { meId: me.id });
+				}
+
+				const config = loadConfig();
+				const defaultTag: string | null = config.defaultTag?.tag;
+				if (defaultTag == null) {
 					qb.orWhere('(note.visibility = \'public\') AND (note.userHost IS NULL)');
+				} else {
+					qb.orWhere(`(note.visibility = 'public') AND (:t <@ note.tags`, { t: normalizeForSearch(defaultTag) });
 				}
 			}))
 			.innerJoinAndSelect('note.user', 'user')
diff --git a/packages/backend/src/server/api/endpoints/notes/local-timeline.ts b/packages/backend/src/server/api/endpoints/notes/local-timeline.ts
index 0b48f2c78b..71e1fa6dad 100644
--- a/packages/backend/src/server/api/endpoints/notes/local-timeline.ts
+++ b/packages/backend/src/server/api/endpoints/notes/local-timeline.ts
@@ -16,6 +16,8 @@ import { QueryService } from '@/core/QueryService.js';
 import { MiLocalUser } from '@/models/User.js';
 import { FanoutTimelineEndpointService } from '@/core/FanoutTimelineEndpointService.js';
 import { ApiError } from '../../error.js';
+import { normalizeForSearch } from '@/misc/normalize-for-search.js';
+import { loadConfig } from '@/config.js';
 
 export const meta = {
 	tags: ['notes'],
@@ -146,9 +148,19 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 		withFiles: boolean,
 		withReplies: boolean,
 	}, me: MiLocalUser | null) {
+		const config = loadConfig();
+		const defaultTag: string | null = config.defaultTag?.tag;
 		const query = this.queryService.makePaginationQuery(this.notesRepository.createQueryBuilder('note'),
 			ps.sinceId, ps.untilId)
-			.andWhere('(note.visibility = \'public\') AND (note.userHost IS NULL) AND (note.channelId IS NULL)')
+			.andWhere(new Brackets(qb => {
+				qb.andWhere('note.visibility = \'public\'');
+				qb.andWhere('note.channelId IS NULL');
+				if (defaultTag == null) {
+					qb.andWhere('note.userHost IS NULL');
+				} else {
+					qb.andWhere(`:t <@ note.tags`, { t: normalizeForSearch(defaultTag) });
+				}
+			}))
 			.innerJoinAndSelect('note.user', 'user')
 			.leftJoinAndSelect('note.reply', 'reply')
 			.leftJoinAndSelect('note.renote', 'renote')
diff --git a/packages/backend/src/server/api/stream/channels/hybrid-timeline.ts b/packages/backend/src/server/api/stream/channels/hybrid-timeline.ts
index 75bd13221f..ee43cb9e44 100644
--- a/packages/backend/src/server/api/stream/channels/hybrid-timeline.ts
+++ b/packages/backend/src/server/api/stream/channels/hybrid-timeline.ts
@@ -12,6 +12,8 @@ import { RoleService } from '@/core/RoleService.js';
 import { isRenotePacked, isQuotePacked } from '@/misc/is-renote.js';
 import type { JsonObject } from '@/misc/json-value.js';
 import Channel, { type MiChannelService } from '../channel.js';
+import { normalizeForSearch } from '@/misc/normalize-for-search.js';
+import { loadConfig } from '@/config.js';
 
 class HybridTimelineChannel extends Channel {
 	public readonly chName = 'hybridTimeline';
@@ -21,6 +23,7 @@ class HybridTimelineChannel extends Channel {
 	private withRenotes: boolean;
 	private withReplies: boolean;
 	private withFiles: boolean;
+	private defaultTag: string | null;
 
 	constructor(
 		private metaService: MetaService,
@@ -42,6 +45,8 @@ class HybridTimelineChannel extends Channel {
 		this.withRenotes = !!(params.withRenotes ?? true);
 		this.withReplies = !!(params.withReplies ?? false);
 		this.withFiles = !!(params.withFiles ?? false);
+		const config = loadConfig();
+		this.defaultTag = config.defaultTag?.tag;
 
 		// Subscribe events
 		this.subscriber.on('notesStream', this.onNote);
@@ -49,6 +54,11 @@ class HybridTimelineChannel extends Channel {
 
 	@bindThis
 	private async onNote(note: Packed<'Note'>) {
+		let matched = false;
+		if (this.defaultTag != null) {
+			const noteTags = note.tags ? note.tags.map((t: string) => t.toLowerCase()) : [];
+			matched = noteTags.includes(normalizeForSearch(this.defaultTag));
+		}
 		const isMe = this.user!.id === note.userId;
 
 		if (this.withFiles && (note.fileIds == null || note.fileIds.length === 0)) return;
@@ -60,7 +70,7 @@ class HybridTimelineChannel extends Channel {
 		if (!(
 			(note.channelId == null && isMe) ||
 			(note.channelId == null && Object.hasOwn(this.following, note.userId)) ||
-			(note.channelId == null && (note.user.host == null && note.visibility === 'public')) ||
+			(note.channelId == null && ((note.user.host == null || matched) && note.visibility === 'public')) ||
 			(note.channelId != null && this.followingChannels.has(note.channelId))
 		)) return;
 
diff --git a/packages/backend/src/server/api/stream/channels/local-timeline.ts b/packages/backend/src/server/api/stream/channels/local-timeline.ts
index 491029f5de..daf5827029 100644
--- a/packages/backend/src/server/api/stream/channels/local-timeline.ts
+++ b/packages/backend/src/server/api/stream/channels/local-timeline.ts
@@ -12,6 +12,8 @@ import { RoleService } from '@/core/RoleService.js';
 import { isQuotePacked, isRenotePacked } from '@/misc/is-renote.js';
 import type { JsonObject } from '@/misc/json-value.js';
 import Channel, { type MiChannelService } from '../channel.js';
+import { normalizeForSearch } from '@/misc/normalize-for-search.js';
+import { loadConfig } from '@/config.js';
 
 class LocalTimelineChannel extends Channel {
 	public readonly chName = 'localTimeline';
@@ -20,6 +22,7 @@ class LocalTimelineChannel extends Channel {
 	private withRenotes: boolean;
 	private withReplies: boolean;
 	private withFiles: boolean;
+	private defaultTag: string | null;
 
 	constructor(
 		private metaService: MetaService,
@@ -41,6 +44,8 @@ class LocalTimelineChannel extends Channel {
 		this.withRenotes = !!(params.withRenotes ?? true);
 		this.withReplies = !!(params.withReplies ?? false);
 		this.withFiles = !!(params.withFiles ?? false);
+		const config = loadConfig();
+		this.defaultTag = config.defaultTag?.tag;
 
 		// Subscribe events
 		this.subscriber.on('notesStream', this.onNote);
@@ -50,7 +55,12 @@ class LocalTimelineChannel extends Channel {
 	private async onNote(note: Packed<'Note'>) {
 		if (this.withFiles && (note.fileIds == null || note.fileIds.length === 0)) return;
 
-		if (note.user.host !== null) return;
+		if (this.defaultTag == null) {
+			if (note.user.host !== null) return;
+		} else {
+			const noteTags = note.tags ? note.tags.map((t: string) => t.toLowerCase()) : [];
+			if (!noteTags.includes(normalizeForSearch(this.defaultTag))) return;
+		}
 		if (note.visibility !== 'public') return;
 		if (note.channelId != null) return;