diff --git a/src/main/java/postman/bottler/letter/controller/ReplyLetterController.java b/src/main/java/postman/bottler/letter/controller/ReplyLetterController.java
index 262716a6..dbd8a3d2 100644
--- a/src/main/java/postman/bottler/letter/controller/ReplyLetterController.java
+++ b/src/main/java/postman/bottler/letter/controller/ReplyLetterController.java
@@ -21,6 +21,7 @@
 import postman.bottler.letter.dto.request.ReplyLetterDeleteRequestDTO;
 import postman.bottler.letter.dto.request.ReplyLetterRequestDTO;
 import postman.bottler.letter.dto.response.PageResponseDTO;
+import postman.bottler.letter.dto.response.ReplyLetterDetailResponseDTO;
 import postman.bottler.letter.dto.response.ReplyLetterHeadersResponseDTO;
 import postman.bottler.letter.dto.response.ReplyLetterResponseDTO;
 import postman.bottler.letter.exception.InvalidPageRequestException;
@@ -84,12 +85,13 @@ public ApiResponse<PageResponseDTO<ReplyLetterHeadersResponseDTO>> getReplyForLe
             description = "지정된 답장 편지의 ID에 대한 상세 정보를 반환합니다."
     )
     @GetMapping("/detail/{replyLetterId}")
-    public ApiResponse<ReplyLetterResponseDTO> getReplyLetter(
+    public ApiResponse<ReplyLetterDetailResponseDTO> getReplyLetter(
             @PathVariable Long replyLetterId,
             @AuthenticationPrincipal CustomUserDetails userDetails
     ) {
         letterBoxService.validateLetterInUserBox(replyLetterId, userDetails.getUserId());
-        ReplyLetterResponseDTO result = letterReplyService.getReplyLetterDetail(replyLetterId);
+        ReplyLetterDetailResponseDTO result = letterReplyService.getReplyLetterDetail(replyLetterId,
+                userDetails.getUserId());
         return ApiResponse.onSuccess(result);
     }
 
diff --git a/src/main/java/postman/bottler/letter/dto/response/ReplyLetterDetailResponseDTO.java b/src/main/java/postman/bottler/letter/dto/response/ReplyLetterDetailResponseDTO.java
new file mode 100644
index 00000000..e34a3d72
--- /dev/null
+++ b/src/main/java/postman/bottler/letter/dto/response/ReplyLetterDetailResponseDTO.java
@@ -0,0 +1,26 @@
+package postman.bottler.letter.dto.response;
+
+import java.time.LocalDateTime;
+import postman.bottler.letter.domain.ReplyLetter;
+
+public record ReplyLetterDetailResponseDTO(
+        Long replyLetterId,
+        String content,
+        String font,
+        String paper,
+        String label,
+        boolean isReplied,
+        LocalDateTime createdAt
+) {
+    public static ReplyLetterDetailResponseDTO from(ReplyLetter replyLetter, boolean isReplied) {
+        return new ReplyLetterDetailResponseDTO(
+                replyLetter.getId(),
+                replyLetter.getContent(),
+                replyLetter.getFont(),
+                replyLetter.getPaper(),
+                replyLetter.getLabel(),
+                isReplied,
+                replyLetter.getCreatedAt()
+        );
+    }
+}
diff --git a/src/main/java/postman/bottler/letter/infra/ReplyLetterJpaRepository.java b/src/main/java/postman/bottler/letter/infra/ReplyLetterJpaRepository.java
index f44f9799..0b1914b9 100644
--- a/src/main/java/postman/bottler/letter/infra/ReplyLetterJpaRepository.java
+++ b/src/main/java/postman/bottler/letter/infra/ReplyLetterJpaRepository.java
@@ -26,4 +26,6 @@ public interface ReplyLetterJpaRepository extends JpaRepository<ReplyLetterEntit
     void blockById(Long id);
 
     boolean existsByLetterIdAndSenderId(Long letterId, Long senderId);
+
+    boolean existsByIdAndSenderId(Long id, Long senderId);
 }
diff --git a/src/main/java/postman/bottler/letter/infra/ReplyLetterRepositoryImpl.java b/src/main/java/postman/bottler/letter/infra/ReplyLetterRepositoryImpl.java
index da2f71ef..13c984b1 100644
--- a/src/main/java/postman/bottler/letter/infra/ReplyLetterRepositoryImpl.java
+++ b/src/main/java/postman/bottler/letter/infra/ReplyLetterRepositoryImpl.java
@@ -50,4 +50,9 @@ public void blockReplyLetterById(Long replyLetterId) {
     public boolean existsByLetterIdAndSenderId(Long letterId, Long senderId) {
         return replyLetterJpaRepository.existsByLetterIdAndSenderId(letterId, senderId);
     }
+
+    @Override
+    public boolean existsByIdAndSenderId(Long replyLetterId, Long userId) {
+        return replyLetterJpaRepository.existsByIdAndSenderId(replyLetterId, userId);
+    }
 }
diff --git a/src/main/java/postman/bottler/letter/service/ReplyLetterRepository.java b/src/main/java/postman/bottler/letter/service/ReplyLetterRepository.java
index 36791ec1..bdc3246f 100644
--- a/src/main/java/postman/bottler/letter/service/ReplyLetterRepository.java
+++ b/src/main/java/postman/bottler/letter/service/ReplyLetterRepository.java
@@ -18,4 +18,6 @@ public interface ReplyLetterRepository {
     void blockReplyLetterById(Long replyLetterId);
 
     boolean existsByLetterIdAndSenderId(Long letterId, Long senderId);
+
+    boolean existsByIdAndSenderId(Long replyLetterId, Long userId);
 }
diff --git a/src/main/java/postman/bottler/letter/service/ReplyLetterService.java b/src/main/java/postman/bottler/letter/service/ReplyLetterService.java
index 0fef4a3f..710ce9e6 100644
--- a/src/main/java/postman/bottler/letter/service/ReplyLetterService.java
+++ b/src/main/java/postman/bottler/letter/service/ReplyLetterService.java
@@ -16,6 +16,7 @@
 import postman.bottler.letter.dto.ReceiverDTO;
 import postman.bottler.letter.dto.request.PageRequestDTO;
 import postman.bottler.letter.dto.request.ReplyLetterRequestDTO;
+import postman.bottler.letter.dto.response.ReplyLetterDetailResponseDTO;
 import postman.bottler.letter.dto.response.ReplyLetterHeadersResponseDTO;
 import postman.bottler.letter.dto.response.ReplyLetterResponseDTO;
 import postman.bottler.letter.exception.DuplicateReplyLetterException;
@@ -81,9 +82,10 @@ public Page<ReplyLetterHeadersResponseDTO> getReplyLetterHeadersById(
     }
 
     @Transactional(readOnly = true)
-    public ReplyLetterResponseDTO getReplyLetterDetail(Long replyLetterId) {
+    public ReplyLetterDetailResponseDTO getReplyLetterDetail(Long replyLetterId, Long userId) {
+        boolean isReplied = replyLetterRepository.existsByIdAndSenderId(replyLetterId, userId);
         ReplyLetter replyLetter = findReplyLetter(replyLetterId);
-        return ReplyLetterResponseDTO.from(replyLetter);
+        return ReplyLetterDetailResponseDTO.from(replyLetter, isReplied);
     }
 
     @Transactional