diff --git a/src/main/java/inu/timetable/controller/TimetableController.java b/src/main/java/inu/timetable/controller/TimetableController.java
index 1a1cb67..befc427 100644
--- a/src/main/java/inu/timetable/controller/TimetableController.java
+++ b/src/main/java/inu/timetable/controller/TimetableController.java
@@ -33,9 +33,9 @@ public ResponseEntity<?> addSubjectToTimetable(@RequestBody Map<String, Object>
     }
     
     @DeleteMapping("/remove")
-    public ResponseEntity<?> removeSubjectFromTimetable(@RequestParam Long userId, @RequestParam Long subjectId) {
+    public ResponseEntity<?> removeSubjectFromTimetable(@RequestParam Long userId, @RequestParam Long subjectId, @RequestParam String semester) {
         try {
-            timetableService.removeSubjectFromTimetable(userId, subjectId);
+            timetableService.removeSubjectFromTimetable(userId, subjectId, semester);
             return ResponseEntity.ok(Map.of("message", "과목이 시간표에서 제거되었습니다."));
             
         } catch (RuntimeException e) {
@@ -46,7 +46,7 @@ public ResponseEntity<?> removeSubjectFromTimetable(@RequestParam Long userId, @
     @GetMapping("/user/{userId}")
     public ResponseEntity<List<UserTimetable>> getUserTimetable(
             @PathVariable Long userId,
-            @RequestParam(required = false) String semester) {
+            @RequestParam String semester) {
         
         List<UserTimetable> timetable = timetableService.getUserTimetable(userId, semester);
         return ResponseEntity.ok(timetable);
@@ -57,9 +57,10 @@ public ResponseEntity<?> updateMemo(@RequestBody Map<String, Object> request) {
         try {
             Long userId = Long.valueOf(request.get("userId").toString());
             Long subjectId = Long.valueOf(request.get("subjectId").toString());
+            String semester = (String) request.get("semester");
             String memo = (String) request.get("memo");
             
-            UserTimetable userTimetable = timetableService.updateMemo(userId, subjectId, memo);
+            UserTimetable userTimetable = timetableService.updateMemo(userId, subjectId, semester, memo);
             return ResponseEntity.ok(userTimetable);
             
         } catch (RuntimeException e) {
@@ -68,7 +69,7 @@ public ResponseEntity<?> updateMemo(@RequestBody Map<String, Object> request) {
     }
     
     @DeleteMapping("/clear")
-    public ResponseEntity<?> clearTimetable(@RequestParam Long userId, @RequestParam(required = false) String semester) {
+    public ResponseEntity<?> clearTimetable(@RequestParam Long userId, @RequestParam String semester) {
         try {
             timetableService.removeAllSubjectsFromTimetable(userId, semester);
             String message = semester != null && !semester.isEmpty() 
diff --git a/src/main/java/inu/timetable/controller/WishlistController.java b/src/main/java/inu/timetable/controller/WishlistController.java
index ee34af7..4b41e59 100644
--- a/src/main/java/inu/timetable/controller/WishlistController.java
+++ b/src/main/java/inu/timetable/controller/WishlistController.java
@@ -38,9 +38,9 @@ public ResponseEntity<?> addToWishlist(@RequestBody Map<String, Object> request)
     }
     
     @DeleteMapping("/remove")
-    public ResponseEntity<?> removeFromWishlist(@RequestParam Long userId, @RequestParam Long subjectId) {
+    public ResponseEntity<?> removeFromWishlist(@RequestParam Long userId, @RequestParam Long subjectId, @RequestParam String semester) {
         try {
-            wishlistService.removeFromWishlist(userId, subjectId);
+            wishlistService.removeFromWishlist(userId, subjectId, semester);
             return ResponseEntity.ok(Map.of("message", "위시리스트에서 제거되었습니다."));
             
         } catch (RuntimeException e) {
@@ -75,9 +75,10 @@ public ResponseEntity<?> updatePriority(@RequestBody Map<String, Object> request
         try {
             Long userId = Long.valueOf(request.get("userId").toString());
             Long subjectId = Long.valueOf(request.get("subjectId").toString());
+            String semester = (String) request.get("semester");
             Integer priority = Integer.valueOf(request.get("priority").toString());
             
-            wishlistService.updatePriority(userId, subjectId, priority);
+            wishlistService.updatePriority(userId, subjectId, semester, priority);
             return ResponseEntity.ok(Map.of("message", "우선순위가 업데이트되었습니다."));
             
         } catch (RuntimeException e) {
@@ -90,9 +91,10 @@ public ResponseEntity<?> updateRequired(@RequestBody Map<String, Object> request
         try {
             Long userId = Long.valueOf(request.get("userId").toString());
             Long subjectId = Long.valueOf(request.get("subjectId").toString());
+            String semester = (String) request.get("semester");
             Boolean isRequired = Boolean.valueOf(request.get("isRequired").toString());
             
-            wishlistService.updateRequired(userId, subjectId, isRequired);
+            wishlistService.updateRequired(userId, subjectId, semester, isRequired);
             return ResponseEntity.ok(Map.of("message", "필수 과목 설정이 업데이트되었습니다."));
             
         } catch (RuntimeException e) {
diff --git a/src/main/java/inu/timetable/repository/UserTimetableRepository.java b/src/main/java/inu/timetable/repository/UserTimetableRepository.java
index 2a18fed..6b94283 100644
--- a/src/main/java/inu/timetable/repository/UserTimetableRepository.java
+++ b/src/main/java/inu/timetable/repository/UserTimetableRepository.java
@@ -2,6 +2,7 @@
 
 import inu.timetable.entity.UserTimetable;
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Modifying;
 import org.springframework.data.jpa.repository.Query;
 import org.springframework.data.repository.query.Param;
 import org.springframework.stereotype.Repository;
@@ -16,8 +17,12 @@ public interface UserTimetableRepository extends JpaRepository<UserTimetable, Lo
     
     List<UserTimetable> findByUserId(Long userId);
     
-    Optional<UserTimetable> findByUserIdAndSubjectId(Long userId, Long subjectId);
+    Optional<UserTimetable> findByUserIdAndSubjectIdAndSemester(Long userId, Long subjectId, String semester);
     
+    @Modifying(clearAutomatically = true)
+    @Query("DELETE FROM UserTimetable ut WHERE ut.user.id = :userId AND ut.subject.id = :subjectId AND ut.semester = :semester")
+    int deleteByUserIdAndSubjectIdAndSemester(@Param("userId") Long userId, @Param("subjectId") Long subjectId, @Param("semester") String semester);
+
     @Query("SELECT DISTINCT ut FROM UserTimetable ut JOIN FETCH ut.subject s WHERE ut.user.id = :userId AND ut.semester = :semester")
     List<UserTimetable> findByUserIdAndSemesterWithSubjectAndSchedules(@Param("userId") Long userId, @Param("semester") String semester);
 }
\ No newline at end of file
diff --git a/src/main/java/inu/timetable/repository/WishlistRepository.java b/src/main/java/inu/timetable/repository/WishlistRepository.java
index 51efa9d..19e0529 100644
--- a/src/main/java/inu/timetable/repository/WishlistRepository.java
+++ b/src/main/java/inu/timetable/repository/WishlistRepository.java
@@ -22,10 +22,10 @@ public interface WishlistRepository extends JpaRepository<WishlistItem, Long> {
            "ORDER BY w.priority")
     List<WishlistItem> findByUserIdAndSemesterWithSubjectAndSchedules(@Param("userId") Long userId, @Param("semester") String semester);
     
-    @Query("SELECT w FROM WishlistItem w JOIN FETCH w.subject s WHERE w.user.id = :userId AND s.id = :subjectId")
-    Optional<WishlistItem> findByUserIdAndSubjectId(@Param("userId") Long userId, @Param("subjectId") Long subjectId);
+    @Query("SELECT w FROM WishlistItem w JOIN FETCH w.subject s WHERE w.user.id = :userId AND s.id = :subjectId AND w.semester = :semester")
+    Optional<WishlistItem> findByUserIdAndSubjectIdAndSemester(@Param("userId") Long userId, @Param("subjectId") Long subjectId, @Param("semester") String semester);
     
-    @Modifying
-    @Query("DELETE FROM WishlistItem w WHERE w.user.id = :userId AND w.subject.id = :subjectId")
-    void deleteByUserIdAndSubjectId(@Param("userId") Long userId, @Param("subjectId") Long subjectId);
+    @Modifying(clearAutomatically = true)
+    @Query("DELETE FROM WishlistItem w WHERE w.user.id = :userId AND w.subject.id = :subjectId AND w.semester = :semester")
+    int deleteByUserIdAndSubjectIdAndSemester(@Param("userId") Long userId, @Param("subjectId") Long subjectId, @Param("semester") String semester);
 }
\ No newline at end of file
diff --git a/src/main/java/inu/timetable/service/TimetableService.java b/src/main/java/inu/timetable/service/TimetableService.java
index 7ab94ce..0b0cfcb 100644
--- a/src/main/java/inu/timetable/service/TimetableService.java
+++ b/src/main/java/inu/timetable/service/TimetableService.java
@@ -8,12 +8,20 @@
 import inu.timetable.repository.UserTimetableRepository;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
 
 import java.util.List;
 import java.util.Optional;
 
 @Service
 public class TimetableService {
+
+    private String requireSemester(String semester) {
+        if (semester == null || semester.isBlank()) {
+            throw new RuntimeException("학기(semester)는 필수입니다.");
+        }
+        return semester;
+    }
     
     private final UserTimetableRepository userTimetableRepository;
     private final UserRepository userRepository;
@@ -36,13 +44,14 @@ public UserTimetable addSubjectToTimetable(Long userId, Long subjectId, String s
             .orElseThrow(() -> new RuntimeException("과목을 찾을 수 없습니다."));
         
         // 이미 추가된 과목인지 확인
-        Optional<UserTimetable> existing = userTimetableRepository.findByUserIdAndSubjectId(userId, subjectId);
+        String sem = requireSemester(semester);
+        Optional<UserTimetable> existing = userTimetableRepository.findByUserIdAndSubjectIdAndSemester(userId, subjectId, sem);
         if (existing.isPresent()) {
             throw new RuntimeException("이미 시간표에 추가된 과목입니다.");
         }
         
         // 시간표 겹침 확인
-        List<UserTimetable> currentTimetable = userTimetableRepository.findByUserIdAndSemesterWithSubjectAndSchedules(userId, semester);
+        List<UserTimetable> currentTimetable = userTimetableRepository.findByUserIdAndSemesterWithSubjectAndSchedules(userId, sem);
         if (hasTimeConflict(currentTimetable, subject)) {
             throw new RuntimeException("시간표가 겹치는 과목이 있습니다.");
         }
@@ -50,18 +59,20 @@ public UserTimetable addSubjectToTimetable(Long userId, Long subjectId, String s
         UserTimetable userTimetable = UserTimetable.builder()
             .user(user)
             .subject(subject)
-            .semester(semester)
+            .semester(sem)
             .memo(memo)
             .build();
             
         return userTimetableRepository.save(userTimetable);
     }
     
-    public void removeSubjectFromTimetable(Long userId, Long subjectId) {
-        UserTimetable userTimetable = userTimetableRepository.findByUserIdAndSubjectId(userId, subjectId)
-            .orElseThrow(() -> new RuntimeException("시간표에서 해당 과목을 찾을 수 없습니다."));
-            
-        userTimetableRepository.delete(userTimetable);
+    @Transactional
+    public void removeSubjectFromTimetable(Long userId, Long subjectId, String semester) {
+        String sem = requireSemester(semester);
+        int deleted = userTimetableRepository.deleteByUserIdAndSubjectIdAndSemester(userId, subjectId, sem);
+        if (deleted == 0) {
+            throw new RuntimeException("시간표에서 해당 과목을 찾을 수 없습니다.");
+        }
     }
     
     public List<UserTimetable> getUserTimetable(Long userId, String semester) {
@@ -72,8 +83,9 @@ public List<UserTimetable> getUserTimetable(Long userId, String semester) {
         }
     }
     
-    public UserTimetable updateMemo(Long userId, Long subjectId, String memo) {
-        UserTimetable userTimetable = userTimetableRepository.findByUserIdAndSubjectId(userId, subjectId)
+    public UserTimetable updateMemo(Long userId, Long subjectId, String semester, String memo) {
+        String sem = requireSemester(semester);
+        UserTimetable userTimetable = userTimetableRepository.findByUserIdAndSubjectIdAndSemester(userId, subjectId, sem)
             .orElseThrow(() -> new RuntimeException("시간표에서 해당 과목을 찾을 수 없습니다."));
             
         userTimetable.setMemo(memo);
diff --git a/src/main/java/inu/timetable/service/WishlistService.java b/src/main/java/inu/timetable/service/WishlistService.java
index 9d446a0..815fb5c 100644
--- a/src/main/java/inu/timetable/service/WishlistService.java
+++ b/src/main/java/inu/timetable/service/WishlistService.java
@@ -15,6 +15,13 @@
 
 @Service
 public class WishlistService {
+
+    private String requireSemester(String semester) {
+        if (semester == null || semester.isBlank()) {
+            throw new RuntimeException("학기(semester)는 필수입니다.");
+        }
+        return semester;
+    }
     
     private final WishlistRepository wishlistRepository;
     private final UserRepository userRepository;
@@ -41,7 +48,8 @@ public WishlistItem addToWishlist(Long userId, Long subjectId, String semester,
             .orElseThrow(() -> new RuntimeException("과목을 찾을 수 없습니다."));
         
         // 이미 위시리스트에 있는지 확인
-        Optional<WishlistItem> existing = wishlistRepository.findByUserIdAndSubjectId(userId, subjectId);
+        String sem = requireSemester(semester);
+        Optional<WishlistItem> existing = wishlistRepository.findByUserIdAndSubjectIdAndSemester(userId, subjectId, sem);
         if (existing.isPresent()) {
             throw new RuntimeException("이미 위시리스트에 추가된 과목입니다.");
         }
@@ -49,7 +57,7 @@ public WishlistItem addToWishlist(Long userId, Long subjectId, String semester,
         WishlistItem wishlistItem = WishlistItem.builder()
             .user(user)
             .subject(subject)
-            .semester(semester)
+            .semester(sem)
             .priority(priority != null ? priority : 3) // 기본 우선순위: 중간
             .isRequired(isRequired != null ? isRequired : false)
             .build();
@@ -58,24 +66,30 @@ public WishlistItem addToWishlist(Long userId, Long subjectId, String semester,
     }
     
     @Transactional
-    public void removeFromWishlist(Long userId, Long subjectId) {
-        wishlistRepository.deleteByUserIdAndSubjectId(userId, subjectId);
+    public void removeFromWishlist(Long userId, Long subjectId, String semester) {
+        String sem = requireSemester(semester);
+        int deleted = wishlistRepository.deleteByUserIdAndSubjectIdAndSemester(userId, subjectId, sem);
+        if (deleted == 0) {
+            throw new RuntimeException("위시리스트에서 해당 과목을 찾을 수 없습니다.");
+        }
     }
     
     public List<WishlistItem> getUserWishlist(Long userId, String semester) {
         return wishlistRepository.findByUserIdAndSemesterWithSubjectAndSchedules(userId, semester);
     }
     
-    public WishlistItem updatePriority(Long userId, Long subjectId, Integer priority) {
-        WishlistItem wishlistItem = wishlistRepository.findByUserIdAndSubjectId(userId, subjectId)
+    public WishlistItem updatePriority(Long userId, Long subjectId, String semester, Integer priority) {
+        String sem = requireSemester(semester);
+        WishlistItem wishlistItem = wishlistRepository.findByUserIdAndSubjectIdAndSemester(userId, subjectId, sem)
             .orElseThrow(() -> new RuntimeException("위시리스트에서 해당 과목을 찾을 수 없습니다."));
             
         wishlistItem.setPriority(priority);
         return wishlistRepository.save(wishlistItem);
     }
     
-    public WishlistItem updateRequired(Long userId, Long subjectId, Boolean isRequired) {
-        WishlistItem wishlistItem = wishlistRepository.findByUserIdAndSubjectId(userId, subjectId)
+    public WishlistItem updateRequired(Long userId, Long subjectId, String semester, Boolean isRequired) {
+        String sem = requireSemester(semester);
+        WishlistItem wishlistItem = wishlistRepository.findByUserIdAndSubjectIdAndSemester(userId, subjectId, sem)
             .orElseThrow(() -> new RuntimeException("위시리스트에서 해당 과목을 찾을 수 없습니다."));
             
         wishlistItem.setIsRequired(isRequired);