diff --git a/apps/gateway/src/conversations/conversations.controller.ts b/apps/gateway/src/conversations/conversations.controller.ts
index 732edff..040be50 100644
--- a/apps/gateway/src/conversations/conversations.controller.ts
+++ b/apps/gateway/src/conversations/conversations.controller.ts
@@ -1,4 +1,5 @@
 import {
+  BadRequestException,
   Body,
   Controller,
   Delete,
@@ -11,6 +12,7 @@ import {
   Param,
   Patch,
   Post,
+  Query,
   UseGuards,
 } from '@nestjs/common';
 import type { Brain } from '@mosaic/brain';
@@ -21,6 +23,7 @@ import {
   CreateConversationDto,
   UpdateConversationDto,
   SendMessageDto,
+  SearchMessagesDto,
 } from './conversations.dto.js';
 
 @Controller('api/conversations')
@@ -33,6 +36,16 @@ export class ConversationsController {
     return this.brain.conversations.findAll(user.id);
   }
 
+  @Get('search')
+  async search(@Query() dto: SearchMessagesDto, @CurrentUser() user: { id: string }) {
+    if (!dto.q || dto.q.trim().length === 0) {
+      throw new BadRequestException('Query parameter "q" is required and must not be empty');
+    }
+    const limit = dto.limit ?? 20;
+    const offset = dto.offset ?? 0;
+    return this.brain.conversations.searchMessages(user.id, dto.q.trim(), limit, offset);
+  }
+
   @Get(':id')
   async findOne(@Param('id') id: string, @CurrentUser() user: { id: string }) {
     const conversation = await this.brain.conversations.findById(id, user.id);
diff --git a/apps/gateway/src/conversations/conversations.dto.ts b/apps/gateway/src/conversations/conversations.dto.ts
index 369ef33..7954df7 100644
--- a/apps/gateway/src/conversations/conversations.dto.ts
+++ b/apps/gateway/src/conversations/conversations.dto.ts
@@ -1,12 +1,35 @@
 import {
   IsBoolean,
   IsIn,
+  IsInt,
   IsObject,
   IsOptional,
   IsString,
   IsUUID,
+  Max,
   MaxLength,
+  Min,
 } from 'class-validator';
+import { Type } from 'class-transformer';
+
+export class SearchMessagesDto {
+  @IsString()
+  @MaxLength(500)
+  q!: string;
+
+  @IsOptional()
+  @Type(() => Number)
+  @IsInt()
+  @Min(1)
+  @Max(100)
+  limit?: number = 20;
+
+  @IsOptional()
+  @Type(() => Number)
+  @IsInt()
+  @Min(0)
+  offset?: number = 0;
+}
 
 export class CreateConversationDto {
   @IsOptional()
diff --git a/packages/brain/src/conversations.ts b/packages/brain/src/conversations.ts
index f1bc88b..4508b96 100644
--- a/packages/brain/src/conversations.ts
+++ b/packages/brain/src/conversations.ts
@@ -1,4 +1,4 @@
-import { eq, and, asc, desc, type Db, conversations, messages } from '@mosaic/db';
+import { eq, and, asc, desc, ilike, type Db, conversations, messages } from '@mosaic/db';
 
 /** Maximum number of conversations returned per list query. */
 const MAX_CONVERSATIONS = 200;
@@ -10,6 +10,15 @@ export type NewConversation = typeof conversations.$inferInsert;
 export type Message = typeof messages.$inferSelect;
 export type NewMessage = typeof messages.$inferInsert;
 
+export interface MessageSearchResult {
+  messageId: string;
+  conversationId: string;
+  conversationTitle: string | null;
+  role: 'user' | 'assistant' | 'system';
+  content: string;
+  createdAt: Date;
+}
+
 export function createConversationsRepo(db: Db) {
   return {
     async findAll(userId: string): Promise<Conversation[]> {
@@ -87,6 +96,35 @@ export function createConversationsRepo(db: Db) {
         .limit(MAX_MESSAGES);
     },
 
+    /**
+     * Search messages by content across all conversations belonging to the user.
+     * Uses ILIKE for case-insensitive substring matching.
+     */
+    async searchMessages(
+      userId: string,
+      query: string,
+      limit: number,
+      offset: number,
+    ): Promise<MessageSearchResult[]> {
+      const rows = await db
+        .select({
+          messageId: messages.id,
+          conversationId: conversations.id,
+          conversationTitle: conversations.title,
+          role: messages.role,
+          content: messages.content,
+          createdAt: messages.createdAt,
+        })
+        .from(messages)
+        .innerJoin(conversations, eq(messages.conversationId, conversations.id))
+        .where(and(eq(conversations.userId, userId), ilike(messages.content, `%${query}%`)))
+        .orderBy(desc(messages.createdAt))
+        .limit(limit)
+        .offset(offset);
+
+      return rows;
+    },
+
     /**
      * Add a message to a conversation, scoped to the given user.
      * Verifies the parent conversation belongs to the user before inserting.
diff --git a/packages/brain/src/index.ts b/packages/brain/src/index.ts
index 4cde737..8bf5516 100644
--- a/packages/brain/src/index.ts
+++ b/packages/brain/src/index.ts
@@ -25,6 +25,7 @@ export {
   type NewConversation,
   type Message,
   type NewMessage,
+  type MessageSearchResult,
 } from './conversations.js';
 export {
   createAgentsRepo,
diff --git a/packages/db/src/index.ts b/packages/db/src/index.ts
index f14fb70..72230ed 100644
--- a/packages/db/src/index.ts
+++ b/packages/db/src/index.ts
@@ -15,4 +15,5 @@ export {
   lt,
   gte,
   lte,
+  ilike,
 } from 'drizzle-orm';