diff --git a/src/teams/teams.service.spec.ts b/src/teams/teams.service.spec.ts
new file mode 100644
index 0000000..86bcc95
--- /dev/null
+++ b/src/teams/teams.service.spec.ts
@@ -0,0 +1,114 @@
+import { BadRequestException } from '@nestjs/common';
+import { Test, TestingModule } from '@nestjs/testing';
+import { getRepositoryToken } from '@nestjs/typeorm';
+import { TeamsService } from './teams.service';
+import { Bounty, Team, TeamMemberSplit } from '../common/entities';
+import { BountyStatus } from '../common/enums';
+
+describe('TeamsService', () => {
+  let service: TeamsService;
+  let teamRepo: { findOne: jest.Mock; save: jest.Mock; create: jest.Mock };
+  let splitRepo: { save: jest.Mock; create: jest.Mock };
+  let bountyRepo: { findOne: jest.Mock; save: jest.Mock };
+
+  beforeEach(async () => {
+    teamRepo = {
+      findOne: jest.fn(),
+      save: jest.fn((t: Partial<Team>) =>
+        Promise.resolve({ id: 'team-1', ...t }),
+      ),
+      create: jest.fn((data: Partial<Team>) => ({ id: 'team-1', ...data })),
+    };
+    splitRepo = {
+      save: jest.fn((s: Partial<TeamMemberSplit>) =>
+        Promise.resolve({ id: 'split-1', ...s }),
+      ),
+      create: jest.fn((data: Partial<TeamMemberSplit>) => ({
+        id: 'split-1',
+        ...data,
+      })),
+    };
+    bountyRepo = {
+      findOne: jest.fn(),
+      save: jest.fn((b: Partial<Bounty>) => Promise.resolve(b)),
+    };
+
+    const module: TestingModule = await Test.createTestingModule({
+      providers: [
+        TeamsService,
+        { provide: getRepositoryToken(Team), useValue: teamRepo },
+        { provide: getRepositoryToken(TeamMemberSplit), useValue: splitRepo },
+        { provide: getRepositoryToken(Bounty), useValue: bountyRepo },
+      ],
+    }).compile();
+
+    service = module.get(TeamsService);
+  });
+
+  describe('assignToBounty', () => {
+    it('assigns a team to an OPEN bounty', async () => {
+      teamRepo.findOne.mockResolvedValue({ id: 'team-1', splits: [] });
+      bountyRepo.findOne.mockResolvedValue({
+        id: 'bounty-1',
+        status: BountyStatus.OPEN,
+        claimedById: null,
+      });
+
+      const updated = await service.assignToBounty('team-1', 'bounty-1');
+      expect(updated.teamId).toBe('team-1');
+      expect(bountyRepo.save).toHaveBeenCalled();
+    });
+
+    it('assigns a team to a FUNDED bounty', async () => {
+      teamRepo.findOne.mockResolvedValue({ id: 'team-1', splits: [] });
+      bountyRepo.findOne.mockResolvedValue({
+        id: 'bounty-1',
+        status: BountyStatus.FUNDED,
+        claimedById: null,
+      });
+
+      const updated = await service.assignToBounty('team-1', 'bounty-1');
+      expect(updated.teamId).toBe('team-1');
+      expect(bountyRepo.save).toHaveBeenCalled();
+    });
+
+    it('rejects assigning a team to a CLAIMED bounty', async () => {
+      teamRepo.findOne.mockResolvedValue({ id: 'team-1', splits: [] });
+      bountyRepo.findOne.mockResolvedValue({
+        id: 'bounty-1',
+        status: BountyStatus.CLAIMED,
+        claimedById: 'user-1',
+      });
+
+      await expect(
+        service.assignToBounty('team-1', 'bounty-1'),
+      ).rejects.toThrow(BadRequestException);
+    });
+
+    it('rejects assigning a team to an IN_REVIEW bounty', async () => {
+      teamRepo.findOne.mockResolvedValue({ id: 'team-1', splits: [] });
+      bountyRepo.findOne.mockResolvedValue({
+        id: 'bounty-1',
+        status: BountyStatus.IN_REVIEW,
+        claimedById: 'user-1',
+      });
+
+      await expect(
+        service.assignToBounty('team-1', 'bounty-1'),
+      ).rejects.toThrow(BadRequestException);
+    });
+
+    it('rejects assigning a team when bounty already has a claimedById', async () => {
+      teamRepo.findOne.mockResolvedValue({ id: 'team-1', splits: [] });
+      bountyRepo.findOne.mockResolvedValue({
+        id: 'bounty-1',
+        status: BountyStatus.FUNDED,
+        claimedById: 'user-1',
+      });
+
+      await expect(
+        service.assignToBounty('team-1', 'bounty-1'),
+      ).rejects.toThrow(BadRequestException);
+    });
+  });
+});
diff --git a/src/teams/teams.service.ts b/src/teams/teams.service.ts
index 28f40b8..c089c64 100644
--- a/src/teams/teams.service.ts
+++ b/src/teams/teams.service.ts
@@ -1,7 +1,12 @@
-import { Injectable, NotFoundException } from '@nestjs/common';
+import {
+  BadRequestException,
+  Injectable,
+  NotFoundException,
+} from '@nestjs/common';
 import { InjectRepository } from '@nestjs/typeorm';
 import { Repository } from 'typeorm';
 import { Bounty, Team, TeamMemberSplit } from '../common/entities';
+import { BountyStatus } from '../common/enums';
 import { CreateTeamDto } from './dto/create-team.dto';
 import { validateSplitPercentages } from './team-split.util';
 
@@ -54,6 +59,22 @@ export class TeamsService {
     await this.findOne(teamId); // ensures team exists
     const bounty = await this.bountyRepo.findOne({ where: { id: bountyId } });
     if (!bounty) throw new NotFoundException(`Bounty ${bountyId} not found`);
+
+    if (
+      bounty.status !== BountyStatus.OPEN &&
+      bounty.status !== BountyStatus.FUNDED
+    ) {
+      throw new BadRequestException(
+        `Cannot assign a team to a bounty in ${bounty.status} status. Assignment is only allowed in OPEN or FUNDED state before claiming.`,
+      );
+    }
+
+    if (bounty.claimedById) {
+      throw new BadRequestException(
+        `Cannot assign a team to a bounty that has already been claimed by contributor ${bounty.claimedById}.`,
+      );
+    }
+
     bounty.teamId = teamId;
     return this.bountyRepo.save(bounty);
   }