diff --git a/src/main/java/com/petlog/diary/controller/DiaryController.java b/src/main/java/com/petlog/diary/controller/DiaryController.java
index 591c1de..73c76da 100644
--- a/src/main/java/com/petlog/diary/controller/DiaryController.java
+++ b/src/main/java/com/petlog/diary/controller/DiaryController.java
@@ -1,10 +1,13 @@
 package com.petlog.diary.controller;
 
+import com.petlog.auth.resolver.Authenticated;
 import com.petlog.common.response.ApiResponse;
 import com.petlog.diary.controller.dto.request.CreateDiaryRequestDto;
 import com.petlog.diary.controller.dto.request.UpdateDiaryRequestDto;
 import com.petlog.diary.controller.dto.response.GetAllDiaryResponseDto;
 import com.petlog.diary.controller.dto.response.GetDiaryResponseDto;
+import com.petlog.diary.service.DiaryService;
+import com.petlog.diary.service.dto.CreateDiaryDto;
 import com.petlog.diary.service.dto.GetDailyDiaryDto;
 import com.petlog.diary.service.dto.GetDiaryInfoDto;
 import com.petlog.docs.DiaryControllerDocs;
@@ -33,11 +36,22 @@
 @RestController
 public class DiaryController implements DiaryControllerDocs {
 
+    private final DiaryService diaryService;
+
     @PostMapping
     public ResponseEntity<ApiResponse<Void>> createDiary(
+        @Authenticated final Long memberId,
         @PathVariable final Long groupId,
         @RequestBody final CreateDiaryRequestDto request
     ) {
+        final CreateDiaryDto dto = new CreateDiaryDto(
+            request.title(),
+            request.content(),
+            request.images(),
+            request.writtenAt()
+        );
+        diaryService.createDiary(memberId, groupId, dto);
+
         return ResponseEntity.ok(
             ApiResponse.success(CREATE_DIARY)
         );
diff --git a/src/main/java/com/petlog/diary/controller/dto/request/CreateDiaryRequestDto.java b/src/main/java/com/petlog/diary/controller/dto/request/CreateDiaryRequestDto.java
index 4447b52..4796f19 100644
--- a/src/main/java/com/petlog/diary/controller/dto/request/CreateDiaryRequestDto.java
+++ b/src/main/java/com/petlog/diary/controller/dto/request/CreateDiaryRequestDto.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 CreateDiaryRequestDto(
     @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/service/DiaryService.java b/src/main/java/com/petlog/diary/service/DiaryService.java
new file mode 100644
index 0000000..b6ab88f
--- /dev/null
+++ b/src/main/java/com/petlog/diary/service/DiaryService.java
@@ -0,0 +1,65 @@
+package com.petlog.diary.service;
+
+import com.petlog.diary.entity.Diary;
+import com.petlog.diary.entity.DiaryImage;
+import com.petlog.diary.repository.DiaryImageRepository;
+import com.petlog.diary.repository.DiaryRepository;
+import com.petlog.diary.service.dto.CreateDiaryDto;
+import com.petlog.member.entity.Member;
+import com.petlog.member.repository.MemberRepository;
+import com.petlog.petgroup.entity.PetGroup;
+import com.petlog.petgroup.entity.PetGroupMember;
+import com.petlog.petgroup.repository.PetGroupMemberRepository;
+import com.petlog.petgroup.repository.PetGroupRepository;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+import java.util.List;
+
+@RequiredArgsConstructor
+@Service
+public class DiaryService {
+
+    private final DiaryRepository diaryRepository;
+    private final DiaryImageRepository diaryImageRepository;
+    private final MemberRepository memberRepository;
+    private final PetGroupRepository petGroupRepository;
+    private final PetGroupMemberRepository petGroupMemberRepository;
+
+    @Transactional
+    public void createDiary(final Long memberId, final Long groupId, final CreateDiaryDto dto) {
+        final Member member = getMember(memberId);
+        final PetGroup petGroup = getPetGroup(groupId);
+        getPetGroupMember(member, petGroup);
+
+        final Diary diary = new Diary(
+            petGroup,
+            member,
+            dto.title(),
+            dto.content(),
+            dto.writtenAt()
+        );
+        diaryRepository.save(diary);
+
+        final List<DiaryImage> images = dto.images().stream()
+            .map(img -> new DiaryImage(diary, img))
+            .toList();
+        diaryImageRepository.saveAll(images);
+    }
+
+    private Member getMember(final Long memberId) {
+        return memberRepository.findById(memberId)
+            .orElseThrow(() -> new IllegalArgumentException("존재하지 않는 회원입니다."));
+    }
+
+    private PetGroup getPetGroup(final Long groupId) {
+        return petGroupRepository.findById(groupId)
+            .orElseThrow(() -> new IllegalArgumentException("존재하지 않는 그룹입니다."));
+    }
+
+    private PetGroupMember getPetGroupMember(final Member member, final PetGroup petGroup) {
+        return petGroupMemberRepository.findByMemberAndPetGroup(member, petGroup)
+            .orElseThrow(() -> new IllegalArgumentException("그룹에 존재하지 않는 회원입니다."));
+    }
+}
diff --git a/src/main/java/com/petlog/diary/service/dto/CreateDiaryDto.java b/src/main/java/com/petlog/diary/service/dto/CreateDiaryDto.java
new file mode 100644
index 0000000..18a88e9
--- /dev/null
+++ b/src/main/java/com/petlog/diary/service/dto/CreateDiaryDto.java
@@ -0,0 +1,14 @@
+package com.petlog.diary.service.dto;
+
+import java.time.LocalDate;
+import java.util.List;
+
+public record CreateDiaryDto(
+
+    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 241ac5d..c054676 100644
--- a/src/main/java/com/petlog/docs/DiaryControllerDocs.java
+++ b/src/main/java/com/petlog/docs/DiaryControllerDocs.java
@@ -1,5 +1,6 @@
 package com.petlog.docs;
 
+import com.petlog.auth.resolver.Authenticated;
 import com.petlog.common.response.ApiResponse;
 import com.petlog.diary.controller.dto.request.CreateDiaryRequestDto;
 import com.petlog.diary.controller.dto.request.UpdateDiaryRequestDto;
@@ -16,7 +17,7 @@ public interface DiaryControllerDocs {
 
     @io.swagger.v3.oas.annotations.responses.ApiResponse(responseCode = "200", description = "일기 생성에 성공하였습니다.")
     @Operation(summary = "일기 생성 API")
-    ResponseEntity<ApiResponse<Void>> createDiary(@PathVariable final Long groupId, @RequestBody final CreateDiaryRequestDto request);
+    ResponseEntity<ApiResponse<Void>> createDiary(@Authenticated final Long memberId, @PathVariable final Long groupId, @RequestBody final CreateDiaryRequestDto request);
 
     @io.swagger.v3.oas.annotations.responses.ApiResponse(responseCode = "200", description = "일기 전체 조회에 성공하였습니다.")
     @Operation(summary = "일기 전체 조회 API")