diff --git a/backend/src/common/common.module.ts b/backend/src/common/common.module.ts
index 7010573d..4ca42823 100644
--- a/backend/src/common/common.module.ts
+++ b/backend/src/common/common.module.ts
@@ -2,7 +2,11 @@ import { Module } from '@nestjs/common';
 import { ConfigModule, ConfigService } from '@nestjs/config';
 import { JwtModule } from '@nestjs/jwt';
 import { PassportModule } from '@nestjs/passport';
+import { TypeOrmModule } from '@nestjs/typeorm';
 import { FilteringService } from './filtering.service';
+import { IdempotencyKey } from './idempotency/idempotency-key.entity';
+import { IdempotencyService } from './idempotency/idempotency.service';
+import { IdempotencyInterceptor } from './idempotency/idempotency.interceptor';
 
 @Module({
   imports: [
@@ -17,8 +21,9 @@ import { FilteringService } from './filtering.service';
         },
       }),
     }),
+    TypeOrmModule.forFeature([IdempotencyKey]),
   ],
-  providers: [FilteringService],
-  exports: [JwtModule, FilteringService],
+  providers: [FilteringService, IdempotencyService, IdempotencyInterceptor],
+  exports: [JwtModule, FilteringService, IdempotencyService, IdempotencyInterceptor],
 })
 export class CommonModule {}
diff --git a/backend/src/common/idempotency/idempotency-key.entity.ts b/backend/src/common/idempotency/idempotency-key.entity.ts
new file mode 100644
index 00000000..8bf8621f
--- /dev/null
+++ b/backend/src/common/idempotency/idempotency-key.entity.ts
@@ -0,0 +1,37 @@
+import {
+  Entity,
+  PrimaryGeneratedColumn,
+  Column,
+  CreateDateColumn,
+  Index,
+} from 'typeorm';
+
+@Entity('idempotency_keys')
+@Index(['key', 'userId'], { unique: true })
+export class IdempotencyKey {
+  @PrimaryGeneratedColumn('uuid')
+  id: string;
+
+  @Column({ type: 'varchar', length: 255 })
+  key: string;
+
+  @Column({ type: 'uuid' })
+  userId: string;
+
+  /** sha256 of method+path+body — detects same-key/different-body reuse */
+  @Column({ name: 'request_hash', type: 'varchar', length: 64 })
+  request_hash: string;
+
+  /** Null while the original request is still executing */
+  @Column({ name: 'status_code', type: 'int', nullable: true })
+  status_code: number | null;
+
+  @Column({ name: 'response_body', type: 'jsonb', nullable: true })
+  response_body: unknown;
+
+  @Column({ name: 'in_progress', type: 'boolean', default: true })
+  in_progress: boolean;
+
+  @CreateDateColumn({ type: 'timestamptz' })
+  created_at: Date;
+}
diff --git a/backend/src/common/idempotency/idempotency.interceptor.ts b/backend/src/common/idempotency/idempotency.interceptor.ts
new file mode 100644
index 00000000..8a8a314f
--- /dev/null
+++ b/backend/src/common/idempotency/idempotency.interceptor.ts
@@ -0,0 +1,72 @@
+import {
+  BadRequestException,
+  CallHandler,
+  ConflictException,
+  ExecutionContext,
+  Injectable,
+  NestInterceptor,
+  UnprocessableEntityException,
+} from '@nestjs/common';
+import { createHash } from 'crypto';
+import type { Request } from 'express';
+import { Observable, of, throwError } from 'rxjs';
+import { catchError, tap } from 'rxjs/operators';
+import { IdempotencyService } from './idempotency.service';
+
+const IDEMPOTENCY_HEADER = 'idempotency-key';
+
+@Injectable()
+export class IdempotencyInterceptor implements NestInterceptor {
+  constructor(private readonly idempotencyService: IdempotencyService) {}
+
+  async intercept(
+    context: ExecutionContext,
+    next: CallHandler,
+  ): Promise<Observable<unknown>> {
+    const request = context.switchToHttp().getRequest<Request & { user: { id: string } }>();
+    const key = request.headers[IDEMPOTENCY_HEADER];
+
+    if (!key || typeof key !== 'string') {
+      throw new BadRequestException(
+        `${IDEMPOTENCY_HEADER} header is required for this request`,
+      );
+    }
+
+    const requestHash = createHash('sha256')
+      .update(`${request.method}:${request.originalUrl}:${JSON.stringify(request.body ?? {})}`)
+      .digest('hex');
+
+    const result = await this.idempotencyService.acquire(
+      key,
+      request.user.id,
+      requestHash,
+    );
+
+    if (!result.acquired) {
+      const { record } = result;
+      if (record.request_hash !== requestHash) {
+        throw new UnprocessableEntityException(
+          'Idempotency-Key was already used with a different request body',
+        );
+      }
+      if (record.in_progress) {
+        throw new ConflictException(
+          'A request with this Idempotency-Key is already in progress',
+        );
+      }
+      return of(record.response_body);
+    }
+
+    const { record } = result;
+    return next.handle().pipe(
+      tap((data) => {
+        const response = context.switchToHttp().getResponse<{ statusCode: number }>();
+        void this.idempotencyService.complete(record.id, response.statusCode, data);
+      }),
+      catchError((err) => {
+        void this.idempotencyService.release(record.id);
+        return throwError(() => err);
+      }),
+    );
+  }
+}
diff --git a/backend/src/common/idempotency/idempotency.service.ts b/backend/src/common/idempotency/idempotency.service.ts
new file mode 100644
index 00000000..f15ac468
--- /dev/null
+++ b/backend/src/common/idempotency/idempotency.service.ts
@@ -0,0 +1,77 @@
+import { Injectable, Logger } from '@nestjs/common';
+import { InjectRepository } from '@nestjs/typeorm';
+import { Cron, CronExpression } from '@nestjs/schedule';
+import { LessThan, Repository } from 'typeorm';
+import { IdempotencyKey } from './idempotency-key.entity';
+
+const POSTGRES_UNIQUE_VIOLATION = '23505';
+const CLEANUP_AGE_MS = 24 * 60 * 60 * 1000;
+
+export type AcquireResult =
+  | { acquired: true; record: IdempotencyKey }
+  | { acquired: false; record: IdempotencyKey };
+
+@Injectable()
+export class IdempotencyService {
+  private readonly logger = new Logger(IdempotencyService.name);
+
+  constructor(
+    @InjectRepository(IdempotencyKey)
+    private readonly repository: Repository<IdempotencyKey>,
+  ) {}
+
+  /**
+   * Atomically claims a key for a user. Relies on the unique (key, userId)
+   * index to detect a concurrent or prior request with the same key.
+   */
+  async acquire(key: string, userId: string, requestHash: string): Promise<AcquireResult> {
+    try {
+      const inserted = await this.repository.save(
+        this.repository.create({
+          key,
+          userId,
+          request_hash: requestHash,
+          in_progress: true,
+        }),
+      );
+      return { acquired: true, record: inserted };
+    } catch (err) {
+      if ((err as { code?: string }).code !== POSTGRES_UNIQUE_VIOLATION) {
+        throw err;
+      }
+      const existing = await this.repository.findOneBy({ key, userId });
+      if (!existing) {
+        throw err;
+      }
+      return { acquired: false, record: existing };
+    }
+  }
+
+  async complete(
+    id: string,
+    statusCode: number,
+    responseBody: unknown,
+  ): Promise<void> {
+    await this.repository.update(id, {
+      status_code: statusCode,
+      response_body: responseBody as object,
+      in_progress: false,
+    });
+  }
+
+  /** Frees the key so the client can safely retry after a failed handler. */
+  async release(id: string): Promise<void> {
+    await this.repository.delete(id);
+  }
+
+  @Cron(CronExpression.EVERY_HOUR)
+  async cleanupExpiredKeys(): Promise<void> {
+    const cutoff = new Date(Date.now() - CLEANUP_AGE_MS);
+    const { affected } = await this.repository.delete({
+      created_at: LessThan(cutoff),
+    });
+    if (affected) {
+      this.logger.log(`Cleaned up ${affected} expired idempotency key(s)`);
+    }
+  }
+}
diff --git a/backend/src/common/idempotency/idempotent.decorator.ts b/backend/src/common/idempotency/idempotent.decorator.ts
new file mode 100644
index 00000000..4f3d7b3a
--- /dev/null
+++ b/backend/src/common/idempotency/idempotent.decorator.ts
@@ -0,0 +1,12 @@
+import { applyDecorators, SetMetadata, UseInterceptors } from '@nestjs/common';
+import { IdempotencyInterceptor } from './idempotency.interceptor';
+
+export const IDEMPOTENT_KEY = 'idempotent';
+
+/** Marks a write route as requiring an `Idempotency-Key` request header. */
+export function Idempotent() {
+  return applyDecorators(
+    SetMetadata(IDEMPOTENT_KEY, true),
+    UseInterceptors(IdempotencyInterceptor),
+  );
+}
diff --git a/backend/src/migrations/1776500000000-CreateIdempotencyKeys.ts b/backend/src/migrations/1776500000000-CreateIdempotencyKeys.ts
new file mode 100644
index 00000000..7fb497de
--- /dev/null
+++ b/backend/src/migrations/1776500000000-CreateIdempotencyKeys.ts
@@ -0,0 +1,103 @@
+import {
+  MigrationInterface,
+  QueryRunner,
+  Table,
+  TableIndex,
+  TableForeignKey,
+} from 'typeorm';
+
+export class CreateIdempotencyKeys1776500000000 implements MigrationInterface {
+  public async up(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.createTable(
+      new Table({
+        name: 'idempotency_keys',
+        columns: [
+          {
+            name: 'id',
+            type: 'uuid',
+            isPrimary: true,
+            generationStrategy: 'uuid',
+            default: 'uuid_generate_v4()',
+          },
+          {
+            name: 'key',
+            type: 'varchar',
+            length: '255',
+            isNullable: false,
+          },
+          {
+            name: 'userId',
+            type: 'uuid',
+            isNullable: false,
+          },
+          {
+            // sha256 of method+path+body — detects same-key/different-body reuse
+            name: 'request_hash',
+            type: 'varchar',
+            length: '64',
+            isNullable: false,
+          },
+          {
+            // Null while the original request is still executing
+            name: 'status_code',
+            type: 'int',
+            isNullable: true,
+          },
+          {
+            name: 'response_body',
+            type: 'jsonb',
+            isNullable: true,
+          },
+          {
+            name: 'in_progress',
+            type: 'boolean',
+            default: true,
+            isNullable: false,
+          },
+          {
+            name: 'created_at',
+            type: 'timestamptz',
+            default: 'CURRENT_TIMESTAMP',
+            isNullable: false,
+          },
+        ],
+      }),
+      true,
+    );
+
+    // Enforces one row per (key, user) — the source of the 409/422 checks
+    await queryRunner.createIndex(
+      'idempotency_keys',
+      new TableIndex({
+        name: 'IDX_idempotency_key_user',
+        columnNames: ['key', 'userId'],
+        isUnique: true,
+      }),
+    );
+
+    // Used by the cleanup cron to find expired rows
+    await queryRunner.createIndex(
+      'idempotency_keys',
+      new TableIndex({
+        name: 'IDX_idempotency_created_at',
+        columnNames: ['created_at'],
+      }),
+    );
+
+    await queryRunner.createForeignKey(
+      'idempotency_keys',
+      new TableForeignKey({
+        name: 'FK_idempotency_user',
+        columnNames: ['userId'],
+        referencedTableName: 'users',
+        referencedColumnNames: ['id'],
+        onDelete: 'CASCADE',
+      }),
+    );
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.dropForeignKey('idempotency_keys', 'FK_idempotency_user');
+    await queryRunner.dropTable('idempotency_keys');
+  }
+}
diff --git a/backend/src/predictions/predictions.controller.ts b/backend/src/predictions/predictions.controller.ts
index e3a40f2c..f2446765 100644
--- a/backend/src/predictions/predictions.controller.ts
+++ b/backend/src/predictions/predictions.controller.ts
@@ -33,6 +33,7 @@ import {
 } from './dto/list-market-predictions.dto';
 import { CurrentUser } from '../common/decorators/current-user.decorator';
 import { Public } from '../common/decorators/public.decorator';
+import { Idempotent } from '../common/idempotency/idempotent.decorator';
 import { User } from '../users/entities/user.entity';
 import { Prediction } from './entities/prediction.entity';
 
@@ -44,6 +45,7 @@ export class PredictionsController {
 
   @Post()
   @UseGuards(BanGuard)
+  @Idempotent()
   @HttpCode(HttpStatus.CREATED)
   @ApiOperation({ summary: 'Submit a prediction on a market' })
   @ApiResponse({
@@ -51,11 +53,15 @@ export class PredictionsController {
     description: 'Prediction submitted',
     type: Prediction,
   })
-  @ApiResponse({ status: 400, description: 'Market closed or invalid outcome' })
+  @ApiResponse({ status: 400, description: 'Market closed, invalid outcome, or missing Idempotency-Key' })
   @ApiResponse({ status: 404, description: 'Market not found' })
   @ApiResponse({
     status: 409,
-    description: 'Duplicate prediction on this market',
+    description: 'Duplicate prediction on this market, or a request with the same Idempotency-Key is already in progress',
+  })
+  @ApiResponse({
+    status: 422,
+    description: 'Idempotency-Key reused with a different request body',
   })
   async submit(
     @Body() dto: SubmitPredictionDto,
diff --git a/backend/src/predictions/predictions.module.ts b/backend/src/predictions/predictions.module.ts
index ebe02ef6..cca999f0 100644
--- a/backend/src/predictions/predictions.module.ts
+++ b/backend/src/predictions/predictions.module.ts
@@ -6,6 +6,7 @@ import { PredictionsController } from './predictions.controller';
 import { UsersModule } from '../users/users.module';
 import { MarketsModule } from '../markets/markets.module';
 import { SorobanModule } from '../soroban/soroban.module';
+import { CommonModule } from '../common/common.module';
 import { User } from '../users/entities/user.entity';
 import { Market } from '../markets/entities/market.entity';
 
@@ -15,6 +16,7 @@ import { Market } from '../markets/entities/market.entity';
     UsersModule,
     MarketsModule,
     SorobanModule,
+    CommonModule,
   ],
   controllers: [PredictionsController],
   providers: [PredictionsService],