diff --git a/locales/en-US.yml b/locales/en-US.yml
index 9160d12382..8163ca525b 100644
--- a/locales/en-US.yml
+++ b/locales/en-US.yml
@@ -638,6 +638,8 @@ userSuspended: "This user has been suspended."
 userSilenced: "This user is being silenced."
 yourAccountSuspendedTitle: "This account is suspended"
 yourAccountSuspendedDescription: "This account has been suspended due to breaking the server's terms of services or similar. Contact the administrator if you would like to know a more detailed reason. Please do not create a new account."
+systemAccountTitle: "This is a system account"
+systemAccountDescription: "This account is created and managed automatically by the system, and cannot be logged into."
 tokenRevoked: "Invalid token"
 tokenRevokedDescription: "This token has expired. Please log in again."
 accountDeleted: "Account deleted"
diff --git a/locales/index.d.ts b/locales/index.d.ts
index d8f914767f..fcd64071af 100644
--- a/locales/index.d.ts
+++ b/locales/index.d.ts
@@ -2568,6 +2568,14 @@ export interface Locale extends ILocale {
      * このアカウントは、サーバーの利用規約に違反したなどの理由により、凍結されています。詳細については管理者までお問い合わせください。新しいアカウントを作らないでください。
      */
     "yourAccountSuspendedDescription": string;
+    /**
+     * This is a system account
+     */
+    "systemAccountTitle": string;
+    /**
+     * This account is created and managed automatically by the system, and cannot be logged into.
+     */
+    "systemAccountDescription": string;
     /**
      * トークンが無効です
      */
diff --git a/locales/ja-JP.yml b/locales/ja-JP.yml
index 3e168f5087..7e39515c5c 100644
--- a/locales/ja-JP.yml
+++ b/locales/ja-JP.yml
@@ -638,6 +638,8 @@ userSuspended: "このユーザーは凍結されています。"
 userSilenced: "このユーザーはサイレンスされています。"
 yourAccountSuspendedTitle: "アカウントが凍結されています"
 yourAccountSuspendedDescription: "このアカウントは、サーバーの利用規約に違反したなどの理由により、凍結されています。詳細については管理者までお問い合わせください。新しいアカウントを作らないでください。"
+systemAccountTitle: "This is a system account"
+systemAccountDescription: "This account is created and managed automatically by the system, and cannot be logged into."
 tokenRevoked: "トークンが無効です"
 tokenRevokedDescription: "ログイントークンが失効しています。ログインし直してください。"
 accountDeleted: "アカウントは削除されています"
diff --git a/packages/backend/src/core/DeleteAccountService.ts b/packages/backend/src/core/DeleteAccountService.ts
index 7f1b8f3efb..8408e95863 100644
--- a/packages/backend/src/core/DeleteAccountService.ts
+++ b/packages/backend/src/core/DeleteAccountService.ts
@@ -13,6 +13,7 @@ import { GlobalEventService } from '@/core/GlobalEventService.js';
 import { UserEntityService } from '@/core/entities/UserEntityService.js';
 import { ApRendererService } from '@/core/activitypub/ApRendererService.js';
 import { ModerationLogService } from '@/core/ModerationLogService.js';
+import { isSystemAccount } from '@/misc/is-system-account.js';
 
 @Injectable()
 export class DeleteAccountService {
@@ -38,6 +39,7 @@ export class DeleteAccountService {
 	}, moderator?: MiUser): Promise<void> {
 		const _user = await this.usersRepository.findOneByOrFail({ id: user.id });
 		if (_user.isRoot) throw new Error('cannot delete a root account');
+		if (isSystemAccount(_user)) throw new Error('cannot delete a system account');
 
 		if (moderator != null) {
 			this.moderationLogService.log(moderator, 'deleteAccount', {
diff --git a/packages/backend/src/core/UserSuspendService.ts b/packages/backend/src/core/UserSuspendService.ts
index 7920e58e36..30dcaa6f7d 100644
--- a/packages/backend/src/core/UserSuspendService.ts
+++ b/packages/backend/src/core/UserSuspendService.ts
@@ -15,6 +15,7 @@ import { UserEntityService } from '@/core/entities/UserEntityService.js';
 import { bindThis } from '@/decorators.js';
 import { RelationshipJobData } from '@/queue/types.js';
 import { ModerationLogService } from '@/core/ModerationLogService.js';
+import { isSystemAccount } from '@/misc/is-system-account.js';
 
 @Injectable()
 export class UserSuspendService {
@@ -38,6 +39,8 @@ export class UserSuspendService {
 
 	@bindThis
 	public async suspend(user: MiUser, moderator: MiUser): Promise<void> {
+		if (isSystemAccount(user)) throw new Error('cannot suspend a system account');
+
 		await this.usersRepository.update(user.id, {
 			isSuspended: true,
 		});
diff --git a/packages/backend/src/core/entities/UserEntityService.ts b/packages/backend/src/core/entities/UserEntityService.ts
index 40830f86b4..d465e2cd4c 100644
--- a/packages/backend/src/core/entities/UserEntityService.ts
+++ b/packages/backend/src/core/entities/UserEntityService.ts
@@ -53,6 +53,7 @@ import type { OnModuleInit } from '@nestjs/common';
 import type { NoteEntityService } from './NoteEntityService.js';
 import type { DriveFileEntityService } from './DriveFileEntityService.js';
 import type { PageEntityService } from './PageEntityService.js';
+import { isSystemAccount } from '@/misc/is-system-account.js';
 
 const Ajv = _Ajv.default;
 const ajv = new Ajv();
@@ -614,6 +615,7 @@ export class UserEntityService implements OnModuleInit {
 				backgroundId: user.backgroundId,
 				isModerator: isModerator,
 				isAdmin: isAdmin,
+				isSystem: isSystemAccount(user),
 				injectFeaturedNote: profile!.injectFeaturedNote,
 				receiveAnnouncementEmail: profile!.receiveAnnouncementEmail,
 				alwaysMarkNsfw: profile!.alwaysMarkNsfw,
diff --git a/packages/backend/src/misc/is-system-account.ts b/packages/backend/src/misc/is-system-account.ts
new file mode 100644
index 0000000000..a34882f494
--- /dev/null
+++ b/packages/backend/src/misc/is-system-account.ts
@@ -0,0 +1,16 @@
+/*
+ * SPDX-FileCopyrightText: hazelnoot and other Sharkey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+interface UserLike {
+	readonly username: string;
+	readonly host: string | null;
+}
+
+/**
+ * Checks if the given user represents a system account, such as instance.actor.
+ */
+export function isSystemAccount(user: UserLike): boolean {
+	return user.host == null && user.username.includes('.');
+}
diff --git a/packages/backend/src/models/json-schema/user.ts b/packages/backend/src/models/json-schema/user.ts
index 249b9bba38..24b6c50e93 100644
--- a/packages/backend/src/models/json-schema/user.ts
+++ b/packages/backend/src/models/json-schema/user.ts
@@ -121,6 +121,11 @@ export const packedUserLiteSchema = {
 			nullable: false, optional: true,
 			default: false,
 		},
+		isSystem: {
+			type: 'boolean',
+			nullable: false, optional: true,
+			default: false,
+		},
 		isSilenced: {
 			type: 'boolean',
 			nullable: false, optional: false,
diff --git a/packages/backend/src/server/api/SigninApiService.ts b/packages/backend/src/server/api/SigninApiService.ts
index 6fbcacbc11..d212e3de79 100644
--- a/packages/backend/src/server/api/SigninApiService.ts
+++ b/packages/backend/src/server/api/SigninApiService.ts
@@ -26,6 +26,7 @@ import { RateLimiterService } from './RateLimiterService.js';
 import { SigninService } from './SigninService.js';
 import type { AuthenticationResponseJSON } from '@simplewebauthn/types';
 import type { FastifyReply, FastifyRequest } from 'fastify';
+import { isSystemAccount } from '@/misc/is-system-account.js';
 
 @Injectable()
 export class SigninApiService {
@@ -125,6 +126,12 @@ export class SigninApiService {
 			});
 		}
 
+		if (isSystemAccount(user)) {
+			return error(403, {
+				id: 's8dhsj9s-a93j-493j-ja9k-kas9sj20aml2',
+			});
+		}
+
 		const profile = await this.userProfilesRepository.findOneByOrFail({ userId: user.id });
 
 		if (!user.approved && instance.approvalRequiredForSignup) {
diff --git a/packages/backend/src/server/api/endpoints/admin/reset-password.ts b/packages/backend/src/server/api/endpoints/admin/reset-password.ts
index 828dbae712..e4bb545f5d 100644
--- a/packages/backend/src/server/api/endpoints/admin/reset-password.ts
+++ b/packages/backend/src/server/api/endpoints/admin/reset-password.ts
@@ -11,6 +11,7 @@ import type { UsersRepository, UserProfilesRepository } from '@/models/_.js';
 import { DI } from '@/di-symbols.js';
 import { secureRndstr } from '@/misc/secure-rndstr.js';
 import { ModerationLogService } from '@/core/ModerationLogService.js';
+import { isSystemAccount } from '@/misc/is-system-account.js';
 
 export const meta = {
 	tags: ['admin'],
@@ -63,6 +64,10 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				throw new Error('cannot reset password of root');
 			}
 
+			if (isSystemAccount(user)) {
+				throw new Error('cannot reset password of system account');
+			}
+
 			const passwd = secureRndstr(8);
 
 			// Generate hash of password
diff --git a/packages/backend/src/server/api/endpoints/admin/show-user.ts b/packages/backend/src/server/api/endpoints/admin/show-user.ts
index a7ca7f9547..2ba064b9dd 100644
--- a/packages/backend/src/server/api/endpoints/admin/show-user.ts
+++ b/packages/backend/src/server/api/endpoints/admin/show-user.ts
@@ -11,6 +11,7 @@ import { RoleService } from '@/core/RoleService.js';
 import { RoleEntityService } from '@/core/entities/RoleEntityService.js';
 import { IdService } from '@/core/IdService.js';
 import { notificationRecieveConfig } from '@/models/json-schema/user.js';
+import { isSystemAccount } from '@/misc/is-system-account.js';
 
 export const meta = {
 	tags: ['admin'],
@@ -31,6 +32,10 @@ export const meta = {
 				type: 'boolean',
 				optional: false, nullable: false,
 			},
+			approved: {
+				type: 'boolean',
+				optional: false, nullable: false,
+			},
 			autoAcceptFollowed: {
 				type: 'boolean',
 				optional: false, nullable: false,
@@ -111,6 +116,10 @@ export const meta = {
 				type: 'boolean',
 				optional: false, nullable: false,
 			},
+			isSystem: {
+				type: 'boolean',
+				optional: false, nullable: false,
+			},
 			isSilenced: {
 				type: 'boolean',
 				optional: false, nullable: false,
@@ -240,6 +249,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				mutedInstances: profile.mutedInstances,
 				notificationRecieveConfig: profile.notificationRecieveConfig,
 				isModerator: isModerator,
+				isSystem: isSystemAccount(user),
 				isSilenced: isSilenced,
 				isSuspended: user.isSuspended,
 				isHibernated: user.isHibernated,
diff --git a/packages/backend/test/unit/misc/is-system-account.ts b/packages/backend/test/unit/misc/is-system-account.ts
new file mode 100644
index 0000000000..d0ab152630
--- /dev/null
+++ b/packages/backend/test/unit/misc/is-system-account.ts
@@ -0,0 +1,47 @@
+/*
+ * SPDX-FileCopyrightText: hazelnoot and other Sharkey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { isSystemAccount } from '@/misc/is-system-account.js';
+
+describe(isSystemAccount, () => {
+	it('should return true for instance.actor', () => {
+		expect(isSystemAccount({ username: 'instance.actor', host: null })).toBeTruthy();
+	});
+
+	it('should return true for relay.actor', () => {
+		expect(isSystemAccount({ username: 'relay.actor', host: null })).toBeTruthy();
+	});
+
+	it('should return true for any username with a dot', () => {
+		expect(isSystemAccount({ username: 'some.user', host: null })).toBeTruthy();
+		expect(isSystemAccount({ username: 'some.', host: null })).toBeTruthy();
+		expect(isSystemAccount({ username: '.user', host: null })).toBeTruthy();
+		expect(isSystemAccount({ username: '.', host: null })).toBeTruthy();
+	});
+
+	it('should return true for usernames with multiple dots', () => {
+		expect(isSystemAccount({ username: 'some.user.account', host: null })).toBeTruthy();
+		expect(isSystemAccount({ username: '..', host: null })).toBeTruthy();
+	});
+
+	it('should return false for usernames without a dot', () => {
+		expect(isSystemAccount({ username: 'instance_actor', host: null })).toBeFalsy();
+		expect(isSystemAccount({ username: 'instanceactor', host: null })).toBeFalsy();
+		expect(isSystemAccount({ username: 'relay_actor', host: null })).toBeFalsy();
+		expect(isSystemAccount({ username: 'relayactor', host: null })).toBeFalsy();
+		expect(isSystemAccount({ username: '', host: null })).toBeFalsy();
+	});
+
+	it('should return false for users from another instance', () => {
+		expect(isSystemAccount({ username: 'instance.actor', host: 'example.com' })).toBeFalsy();
+		expect(isSystemAccount({ username: 'relay.actor', host: 'example.com' })).toBeFalsy();
+		expect(isSystemAccount({ username: 'some.user', host: 'example.com' })).toBeFalsy();
+		expect(isSystemAccount({ username: 'some.', host: 'example.com' })).toBeFalsy();
+		expect(isSystemAccount({ username: '.user', host: 'example.com' })).toBeFalsy();
+		expect(isSystemAccount({ username: '.', host: 'example.com' })).toBeFalsy();
+		expect(isSystemAccount({ username: 'some.user.account', host: 'example.com' })).toBeFalsy();
+		expect(isSystemAccount({ username: '..', host: 'example.com' })).toBeFalsy();
+	});
+});
diff --git a/packages/frontend/src/components/MkSignin.vue b/packages/frontend/src/components/MkSignin.vue
index 42fa2bf4a7..9813774da3 100644
--- a/packages/frontend/src/components/MkSignin.vue
+++ b/packages/frontend/src/components/MkSignin.vue
@@ -77,6 +77,7 @@ import { misskeyApi } from '@/scripts/misskey-api.js';
 import { query, extractDomain } from '@/scripts/url.js';
 import { login } from '@/account.js';
 import { i18n } from '@/i18n.js';
+import { showSystemAccountDialog } from '@/scripts/show-system-account-dialog.js';
 
 const signing = ref(false);
 const user = ref<Misskey.entities.UserDetailed | null>(null);
@@ -204,6 +205,10 @@ function loginFailed(err: any): void {
 			showSuspendedDialog();
 			break;
 		}
+		case 's8dhsj9s-a93j-493j-ja9k-kas9sj20aml2': {
+			showSystemAccountDialog();
+			break;
+		}
 		case '22d05606-fbcf-421a-a2db-b32610dcfd1b': {
 			os.alert({
 				type: 'error',
diff --git a/packages/frontend/src/pages/admin-user.vue b/packages/frontend/src/pages/admin-user.vue
index 187ec66b42..16a2a7524b 100644
--- a/packages/frontend/src/pages/admin-user.vue
+++ b/packages/frontend/src/pages/admin-user.vue
@@ -24,7 +24,7 @@ SPDX-License-Identifier: AGPL-3.0-only
 					</div>
 				</div>
 
-				<MkInfo v-if="['instance.actor', 'relay.actor'].includes(user.username)">{{ i18n.ts.isSystemAccount }}</MkInfo>
+				<MkInfo v-if="isSystem">{{ i18n.ts.isSystemAccount }}</MkInfo>
 
 				<FormLink v-if="user.host" :to="`/instance-info/${user.host}`">{{ i18n.ts.instanceInfo }}</FormLink>
 
@@ -79,11 +79,11 @@ SPDX-License-Identifier: AGPL-3.0-only
 				<FormSection>
 					<div class="_gaps">
 						<MkSwitch v-model="silenced" @update:modelValue="toggleSilence">{{ i18n.ts.silence }}</MkSwitch>
-						<MkSwitch v-model="suspended" @update:modelValue="toggleSuspend">{{ i18n.ts.suspend }}</MkSwitch>
+						<MkSwitch v-if="!isSystem" v-model="suspended" @update:modelValue="toggleSuspend">{{ i18n.ts.suspend }}</MkSwitch>
 						<MkSwitch v-model="markedAsNSFW" @update:modelValue="toggleNSFW">{{ i18n.ts.markAsNSFW }}</MkSwitch>
 
 						<div>
-							<MkButton v-if="user.host == null" inline style="margin-right: 8px;" @click="resetPassword"><i class="ti ti-key"></i> {{ i18n.ts.resetPassword }}</MkButton>
+							<MkButton v-if="user.host == null && !isSystem" inline style="margin-right: 8px;" @click="resetPassword"><i class="ti ti-key"></i> {{ i18n.ts.resetPassword }}</MkButton>
 						</div>
 
 						<MkFolder>
@@ -114,7 +114,7 @@ SPDX-License-Identifier: AGPL-3.0-only
 							<MkButton v-if="iAmModerator" inline danger style="margin-right: 8px;" @click="unsetUserBanner"><i class="ti ti-photo"></i> {{ i18n.ts.unsetUserBanner }}</MkButton>
 							<MkButton v-if="iAmModerator" inline danger @click="deleteAllFiles"><i class="ph-cloud ph-bold ph-lg"></i> {{ i18n.ts.deleteAllFiles }}</MkButton>
 						</div>
-						<MkButton v-if="$i.isAdmin" inline danger @click="deleteAccount">{{ i18n.ts.deleteAccount }}</MkButton>
+						<MkButton v-if="$i.isAdmin && !isSystem" inline danger @click="deleteAccount">{{ i18n.ts.deleteAccount }}</MkButton>
 					</div>
 				</FormSection>
 			</div>
@@ -227,15 +227,16 @@ const tab = ref(props.initialTab);
 const chartSrc = ref('per-user-notes');
 const user = ref<null | Misskey.entities.UserDetailed>();
 const init = ref<ReturnType<typeof createFetcher>>();
-const info = ref<any>();
+const info = ref<Misskey.entities.AdminShowUserResponse | null>(null);
 const ips = ref<Misskey.entities.AdminGetUserIpsResponse | null>(null);
-const ap = ref<any>(null);
+const ap = ref<Misskey.entities.ApGetResponse | null>(null);
 const moderator = ref(false);
 const silenced = ref(false);
 const approved = ref(false);
 const suspended = ref(false);
 const markedAsNSFW = ref(false);
 const moderationNote = ref('');
+const isSystem = computed(() => info.value?.isSystem ?? false);
 const filesPagination = {
 	endpoint: 'admin/drive/files' as const,
 	limit: 10,
diff --git a/packages/frontend/src/scripts/show-system-account-dialog.ts b/packages/frontend/src/scripts/show-system-account-dialog.ts
new file mode 100644
index 0000000000..3c28d901fc
--- /dev/null
+++ b/packages/frontend/src/scripts/show-system-account-dialog.ts
@@ -0,0 +1,15 @@
+/*
+ * SPDX-FileCopyrightText: hazelnoot and other Sharkey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import * as os from '@/os.js';
+import { i18n } from '@/i18n.js';
+
+export function showSystemAccountDialog(): Promise<void> {
+	return os.alert({
+		type: 'error',
+		title: i18n.ts.systemAccountTitle,
+		text: i18n.ts.systemAccountDescription,
+	});
+}
diff --git a/packages/misskey-js/src/autogen/types.ts b/packages/misskey-js/src/autogen/types.ts
index a82c108d91..6f8e0ca56b 100644
--- a/packages/misskey-js/src/autogen/types.ts
+++ b/packages/misskey-js/src/autogen/types.ts
@@ -3836,6 +3836,8 @@ export type components = {
       isAdmin?: boolean;
       /** @default false */
       isModerator?: boolean;
+      /** @default false */
+      isSystem?: boolean;
       isSilenced: boolean;
       noindex: boolean;
       isBot?: boolean;
@@ -9077,6 +9079,7 @@ export type operations = {
           'application/json': {
             email: string | null;
             emailVerified: boolean;
+            approved: boolean;
             autoAcceptFollowed: boolean;
             noCrawle: boolean;
             preventAiLearning: boolean;
@@ -9216,6 +9219,7 @@ export type operations = {
               }]>;
             };
             isModerator: boolean;
+            isSystem: boolean;
             isSilenced: boolean;
             isSuspended: boolean;
             isHibernated: boolean;