diff --git a/src/main/java/com/petlog/diary/controller/DiaryController.java b/src/main/java/com/petlog/diary/controller/DiaryController.java
index 37b52c1..34a8f78 100644
--- a/src/main/java/com/petlog/diary/controller/DiaryController.java
+++ b/src/main/java/com/petlog/diary/controller/DiaryController.java
@@ -10,6 +10,7 @@
 import com.petlog.diary.service.dto.CreateDiaryDto;
 import com.petlog.diary.service.dto.GetDiaryDto;
 import com.petlog.diary.service.dto.GetDiaryInfoDto;
+import com.petlog.diary.service.dto.UpdateDiaryDto;
 import com.petlog.docs.DiaryControllerDocs;
 import lombok.RequiredArgsConstructor;
 import org.springframework.http.ResponseEntity;
@@ -91,10 +92,19 @@ public ResponseEntity<ApiResponse<GetDiaryResponseDto>> getDiary(
 
     @PatchMapping("/{diaryId}")
     public ResponseEntity<ApiResponse<Void>> updateDiary(
+        @Authenticated final Long memberId,
         @PathVariable final Long groupId,
         @PathVariable final Long diaryId,
         @RequestBody final UpdateDiaryRequestDto request
     ) {
+        final UpdateDiaryDto dto = new UpdateDiaryDto(
+            request.title(),
+            request.content(),
+            request.images(),
+            request.writtenAt()
+        );
+        diaryService.updateDiary(memberId, groupId, diaryId, dto);
+
         return ResponseEntity.ok(
             ApiResponse.success(UPDATE_DIARY)
         );
diff --git a/src/main/java/com/petlog/diary/controller/dto/request/UpdateDiaryRequestDto.java b/src/main/java/com/petlog/diary/controller/dto/request/UpdateDiaryRequestDto.java
index 1fb762a..e3f4a3c 100644
--- a/src/main/java/com/petlog/diary/controller/dto/request/UpdateDiaryRequestDto.java
+++ b/src/main/java/com/petlog/diary/controller/dto/request/UpdateDiaryRequestDto.java
@@ -1,6 +1,8 @@
 package com.petlog.diary.controller.dto.request;
 
+import com.fasterxml.jackson.annotation.JsonFormat;
 import io.swagger.v3.oas.annotations.media.Schema;
+import org.springframework.format.annotation.DateTimeFormat;
 
 import java.time.LocalDate;
 import java.util.List;
@@ -16,6 +18,8 @@ public record UpdateDiaryRequestDto(
     @Schema(description = "0~6장")
     List<String> images,
 
+    @JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy.MM.dd")
+    @DateTimeFormat(pattern = "yyyy.MM.dd")
     LocalDate writtenAt
 
 ) {
diff --git a/src/main/java/com/petlog/diary/entity/Diary.java b/src/main/java/com/petlog/diary/entity/Diary.java
index 96c5900..fe96243 100644
--- a/src/main/java/com/petlog/diary/entity/Diary.java
+++ b/src/main/java/com/petlog/diary/entity/Diary.java
@@ -64,4 +64,14 @@ public Diary(
         this.content = content;
         this.writtenAt = writtenAt;
     }
+
+    public void update(
+        final String title,
+        final String content,
+        final LocalDate writtenAt
+    ) {
+        this.title = title;
+        this.content = content;
+        this.writtenAt = writtenAt;
+    }
 }
diff --git a/src/main/java/com/petlog/diary/repository/DiaryImageRepository.java b/src/main/java/com/petlog/diary/repository/DiaryImageRepository.java
index 3d2aa67..c440036 100644
--- a/src/main/java/com/petlog/diary/repository/DiaryImageRepository.java
+++ b/src/main/java/com/petlog/diary/repository/DiaryImageRepository.java
@@ -1,7 +1,10 @@
 package com.petlog.diary.repository;
 
+import com.petlog.diary.entity.Diary;
 import com.petlog.diary.entity.DiaryImage;
 import org.springframework.data.jpa.repository.JpaRepository;
 
 public interface DiaryImageRepository extends JpaRepository<DiaryImage, Long> {
+
+    void deleteAllByDiary(final Diary diary);
 }
diff --git a/src/main/java/com/petlog/diary/service/DiaryService.java b/src/main/java/com/petlog/diary/service/DiaryService.java
index 6332ec6..e3028db 100644
--- a/src/main/java/com/petlog/diary/service/DiaryService.java
+++ b/src/main/java/com/petlog/diary/service/DiaryService.java
@@ -7,6 +7,7 @@
 import com.petlog.diary.service.dto.CreateDiaryDto;
 import com.petlog.diary.service.dto.GetDiaryDto;
 import com.petlog.diary.service.dto.GetDiaryInfoDto;
+import com.petlog.diary.service.dto.UpdateDiaryDto;
 import com.petlog.member.entity.Member;
 import com.petlog.member.repository.MemberRepository;
 import com.petlog.petgroup.entity.PetGroup;
@@ -86,6 +87,7 @@ public List<GetDiaryInfoDto> getAllDiaries(final Long memberId, final Long group
             .toList();
     }
 
+    @Transactional(readOnly = true)
     public GetDiaryDto getDiary(final Long memberId, final Long groupId, final Long diaryId) {
         final Member member = getMember(memberId);
         final PetGroup petGroup = getPetGroup(groupId);
@@ -108,4 +110,33 @@ private Diary getDiaryDetail(final Long diaryId) {
         return diaryRepository.findById(diaryId)
             .orElseThrow(() -> new IllegalArgumentException("존재하지 않는 일기입니다."));
     }
+
+    @Transactional
+    public void updateDiary(final Long memberId, final Long groupId, final Long diaryId, final UpdateDiaryDto dto) {
+        final Member member = getMember(memberId);
+        final PetGroup petGroup = getPetGroup(groupId);
+        getPetGroupMember(member, petGroup);
+
+        final Diary diary = getDiaryDetail(diaryId);
+        validateIsDairyWriter(member, diary);
+
+        diary.update(
+            dto.title(),
+            dto.content(),
+            dto.writtenAt()
+        );
+
+        diaryImageRepository.deleteAllByDiary(diary);
+
+        final List<DiaryImage> images = dto.images().stream()
+            .map(img -> new DiaryImage(diary, img))
+            .toList();
+        diaryImageRepository.saveAll(images);
+    }
+
+    private void validateIsDairyWriter(final Member member, final Diary diary) {
+        if(!diary.getMember().equals(member)) {
+            throw new IllegalArgumentException("일기 작성자만 수정할 수 있습니다.");
+        }
+    }
 }
diff --git a/src/main/java/com/petlog/diary/service/dto/UpdateDiaryDto.java b/src/main/java/com/petlog/diary/service/dto/UpdateDiaryDto.java
new file mode 100644
index 0000000..aa460b8
--- /dev/null
+++ b/src/main/java/com/petlog/diary/service/dto/UpdateDiaryDto.java
@@ -0,0 +1,14 @@
+package com.petlog.diary.service.dto;
+
+import java.time.LocalDate;
+import java.util.List;
+
+public record UpdateDiaryDto(
+
+    String title,
+    String content,
+    List<String> images,
+    LocalDate writtenAt
+
+) {
+}
diff --git a/src/main/java/com/petlog/docs/DiaryControllerDocs.java b/src/main/java/com/petlog/docs/DiaryControllerDocs.java
index 70309ca..946b99f 100644
--- a/src/main/java/com/petlog/docs/DiaryControllerDocs.java
+++ b/src/main/java/com/petlog/docs/DiaryControllerDocs.java
@@ -29,7 +29,7 @@ public interface DiaryControllerDocs {
 
     @io.swagger.v3.oas.annotations.responses.ApiResponse(responseCode = "200", description = "일기 상세 내용 수정에 성공하였습니다.")
     @Operation(summary = "일기 상세 내용 수정 API")
-    ResponseEntity<ApiResponse<Void>> updateDiary(@PathVariable final Long groupId, @PathVariable final Long diaryId, @RequestBody final UpdateDiaryRequestDto request);
+    ResponseEntity<ApiResponse<Void>> updateDiary(@Authenticated final Long memberId, @PathVariable final Long groupId, @PathVariable final Long diaryId, @RequestBody final UpdateDiaryRequestDto request);
 
     @io.swagger.v3.oas.annotations.responses.ApiResponse(responseCode = "200", description = "일기 삭제가 성공하였습니다.")
     @Operation(summary = "일기 삭제 API")