diff --git a/src/test/java/kusitms/backend/user/application/service/UserApplicationServiceTest.java b/src/test/java/kusitms/backend/user/application/service/UserApplicationServiceTest.java
new file mode 100644
index 0000000..b793070
--- /dev/null
+++ b/src/test/java/kusitms/backend/user/application/service/UserApplicationServiceTest.java
@@ -0,0 +1,174 @@
+package kusitms.backend.user.application.service;
+
+import kusitms.backend.global.exception.CustomException;
+import kusitms.backend.global.redis.DistributedLockManager;
+import kusitms.backend.global.redis.RedisManager;
+import kusitms.backend.user.application.UserApplicationService;
+import kusitms.backend.user.application.dto.request.CheckNicknameRequestDto;
+import kusitms.backend.user.application.dto.request.SendAuthCodeRequestDto;
+import kusitms.backend.user.application.dto.request.VerifyAuthCodeRequestDto;
+import kusitms.backend.user.application.dto.response.AuthTokenResponseDto;
+import kusitms.backend.user.domain.enums.ProviderStatusType;
+import kusitms.backend.user.domain.model.User;
+import kusitms.backend.user.domain.repository.UserRepository;
+import kusitms.backend.user.infra.jwt.JWTUtil;
+import kusitms.backend.user.infra.sms.SmsManager;
+import kusitms.backend.user.status.AuthErrorStatus;
+import kusitms.backend.user.status.UserErrorStatus;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.Mockito.*;
+
+class UserApplicationServiceTest {
+
+    @InjectMocks
+    private UserApplicationService service;
+
+    @Mock
+    private UserRepository userRepository;
+
+    @Mock
+    private JWTUtil jwtUtil;
+
+    @Mock
+    private SmsManager smsManager;
+
+    @Mock
+    private RedisManager redisManager;
+
+    @Mock
+    private DistributedLockManager distributedLockManager;
+
+    @BeforeEach
+    void setUp() {
+        MockitoAnnotations.openMocks(this);
+    }
+
+    /**
+     * 닉네임 중복 확인 성공 테스트.
+     */
+    @Test
+    void testCheckNickname_Success() {
+        // Given
+        String nickname = "newNickname";
+        CheckNicknameRequestDto request = new CheckNicknameRequestDto(nickname);
+        String lockKey = "lock:nickname:" + nickname;
+
+        when(distributedLockManager.acquireLock(lockKey, 5000)).thenReturn(true);
+        when(userRepository.findUserByNickname(nickname)).thenReturn(null);
+
+        // When & Then
+        assertDoesNotThrow(() -> service.checkNickname(request));
+        verify(distributedLockManager, times(1)).releaseLock(lockKey);
+    }
+
+    /**
+     * 닉네임 중복 확인 실패 테스트 (중복된 닉네임).
+     */
+    @Test
+    void testCheckNickname_Failure() {
+        // Given
+        String nickname = "existingNickname";
+        CheckNicknameRequestDto request = new CheckNicknameRequestDto(nickname);
+        String lockKey = "lock:nickname:" + nickname;
+
+        when(distributedLockManager.acquireLock(lockKey, 5000)).thenReturn(true);
+        when(userRepository.findUserByNickname(nickname))
+                .thenReturn(User.toDomain(1L, ProviderStatusType.KAKAO, "providerId", "email@example.com", nickname));
+
+        // When & Then
+        CustomException exception = assertThrows(CustomException.class, () -> service.checkNickname(request));
+        assertEquals(UserErrorStatus._DUPLICATED_NICKNAME, exception.getErrorCode());
+        verify(distributedLockManager, times(1)).releaseLock(lockKey);
+    }
+
+    /**
+     * 인증 코드 발송 테스트.
+     */
+    @Test
+    void testSendAuthCode_Success() {
+        // Given
+        String phoneNumber = "01012345678";
+        SendAuthCodeRequestDto request = new SendAuthCodeRequestDto(phoneNumber);
+
+        // When
+        service.sendAuthCode(request);
+
+        // Then
+        verify(redisManager, times(1)).savePhoneVerificationCode(eq(phoneNumber), anyString());
+        verify(smsManager, times(1)).sendSms(eq(phoneNumber), contains("히트존 인증 코드 번호"));
+    }
+
+    /**
+     * 인증 코드 검증 성공 테스트.
+     */
+    @Test
+    void testVerifyAuthCode_Success() {
+        // Given
+        String phoneNumber = "01012345678";
+        String authCode = "123456";
+        VerifyAuthCodeRequestDto request = new VerifyAuthCodeRequestDto(phoneNumber, authCode);
+
+        when(redisManager.getPhoneVerificationCode(phoneNumber)).thenReturn(authCode);
+
+        // When & Then
+        assertDoesNotThrow(() -> service.verifyAuthCode(request));
+    }
+
+    /**
+     * 인증 코드 검증 실패 테스트 (코드 불일치).
+     */
+    @Test
+    void testVerifyAuthCode_Mismatch() {
+        // Given
+        String phoneNumber = "01012345678";
+        String savedAuthCode = "123456";
+        String wrongAuthCode = "654321";
+        VerifyAuthCodeRequestDto request = new VerifyAuthCodeRequestDto(phoneNumber, wrongAuthCode);
+
+        when(redisManager.getPhoneVerificationCode(phoneNumber)).thenReturn(savedAuthCode);
+
+        // When & Then
+        CustomException exception = assertThrows(CustomException.class, () -> service.verifyAuthCode(request));
+        assertEquals(UserErrorStatus._MISS_MATCH_AUTH_CODE, exception.getErrorCode());
+    }
+
+    /**
+     * Refresh Token 재발급 성공 테스트.
+     */
+    @Test
+    void testReIssueToken_Success() {
+        // Given
+        String refreshToken = "validRefreshToken";
+        Long userId = 1L;
+
+        when(redisManager.validateAndExtractUserId(refreshToken)).thenReturn(userId);
+        when(jwtUtil.generateAccessOrRefreshToken(userId, jwtUtil.getAccessTokenExpirationTime())).thenReturn("newAccessToken");
+        when(jwtUtil.generateAccessOrRefreshToken(userId, jwtUtil.getRefreshTokenExpirationTime())).thenReturn("newRefreshToken");
+
+        // When
+        AuthTokenResponseDto result = service.reIssueToken(refreshToken);
+
+        // Then
+        assertNotNull(result);
+        assertEquals("newRefreshToken", result.refreshToken());
+    }
+
+    /**
+     * Refresh Token 재발급 실패 테스트 (토큰 만료).
+     */
+    @Test
+    void testReIssueToken_Failure() {
+        // Given
+        String expiredRefreshToken = null;
+
+        // When & Then
+        CustomException exception = assertThrows(CustomException.class, () -> service.reIssueToken(expiredRefreshToken));
+        assertEquals(AuthErrorStatus._EXPIRED_REFRESH_TOKEN, exception.getErrorCode());
+    }
+}