diff --git a/src/page/post-detail-page.tsx b/src/page/post-detail-page.tsx
index 1b74e09..edc4a56 100644
--- a/src/page/post-detail-page.tsx
+++ b/src/page/post-detail-page.tsx
@@ -19,6 +19,9 @@ import {
 import { getMyMemberId } from '@shared/utils/auth';
 import { COMMENT_QUERY_OPTIONS } from '@shared/api/domain/comments/query';
 import { queryClient } from '@app/providers/query-client';
+import { COMMENT_MUTATION_OPTIONS } from '@shared/api/domain/comments/query';
+import { useState } from 'react';
+import XIcon from '@shared/assets/icon/x.svg?react';
 
 const handleShare = async () => {
   const url = window.location.href;
@@ -40,6 +43,10 @@ const PostDetailPage = () => {
   const navigate = useNavigate();
   const { feedId } = useParams();
   const numericFeedId = Number(feedId);
+  const [comment, setComment] = useState('');
+  const [parentId, setParentId] = useState<number | null>(null);
+  const [replyTarget, setReplyTarget] = useState<string | null>(null);
+
   const { mutate: applyParticipation } = useMutation({
     ...PARTICIPATION_MUTATION_OPTIONS.APPLY(),
     onSuccess: () => {
@@ -91,6 +98,18 @@ const PostDetailPage = () => {
       });
     },
   });
+  const { mutate: createComment } = useMutation({
+    ...COMMENT_MUTATION_OPTIONS.CREATE(),
+    onSuccess: () => {
+      setComment('');
+      setParentId(null);
+      setReplyTarget(null);
+
+      queryClient.invalidateQueries({
+        queryKey: ['comments', numericFeedId],
+      });
+    },
+  });
 
   const { data, isLoading } = useQuery(
     FEED_QUERY_OPTIONS.DETAIL(numericFeedId),
@@ -142,6 +161,10 @@ const PostDetailPage = () => {
           comments={commentsData ?? []}
           participants={participants}
           isOwner={isOwner}
+          onReply={(commentId, nickname) => {
+            setParentId(commentId);
+            setReplyTarget(nickname ?? null);
+          }}
           onChangeApproval={(participationId, status) => {
             if (status === 'approved') {
               approveParticipation(participationId);
@@ -159,12 +182,46 @@ const PostDetailPage = () => {
           </Button>
         </div>
       )}
-      <div className="flex w-full gap-[1.6rem] py-[1.4rem] px-[2.4rem]">
-        <Input inputSize="sm" placeholder="댓글을 입력해주세요" />
-        <FloatingActionButton
-          mode="inline"
-          icon={<SendIcon width={'2rem'} height={'2rem'} />}
-        />
+      <div className="flex flex-col px-[2.4rem] py-[1.4rem] gap-[0.6rem]">
+        {replyTarget && (
+          <div className="flex justify-between items-center pr-[6rem] text-gray-400 typo-caption">
+            <span>↳{replyTarget}님에게 답글 작성중...</span>
+
+            <button
+              onClick={() => {
+                setParentId(null);
+                setReplyTarget(null);
+              }}
+            >
+              <XIcon width={'2rem'} height={'2rem'} />
+            </button>
+          </div>
+        )}
+
+        <div className="flex w-full gap-[1.6rem]">
+          <Input
+            inputSize="sm"
+            placeholder="댓글을 입력해주세요"
+            value={comment}
+            onChange={(e) => setComment(e.target.value)}
+          />
+
+          <FloatingActionButton
+            mode="inline"
+            icon={<SendIcon width={'2rem'} height={'2rem'} />}
+            onClick={() => {
+              if (!comment.trim()) return;
+
+              createComment({
+                feedId: numericFeedId,
+                body: {
+                  description: comment,
+                  parentId: parentId ?? undefined,
+                },
+              });
+            }}
+          />
+        </div>
       </div>
     </div>
   );
diff --git a/src/shared/api/domain/comments/query.ts b/src/shared/api/domain/comments/query.ts
index 5a5f5bc..9816f7b 100644
--- a/src/shared/api/domain/comments/query.ts
+++ b/src/shared/api/domain/comments/query.ts
@@ -1,12 +1,21 @@
-import { queryOptions } from '@tanstack/react-query';
+import { queryOptions, mutationOptions } from '@tanstack/react-query';
 import { api } from '@shared/api/config/instance';
 import { END_POINT } from '@shared/api/end-point';
-import type { GetCommentsResponse } from '@shared/types/comments/type';
+import type {
+  CreateCommentRequest,
+  GetCommentsResponse,
+} from '@shared/types/comments/type';
 
 const getComments = async (feedId: number) => {
   return api.get(END_POINT.FEED.COMMENTS(feedId)).json<GetCommentsResponse>();
 };
 
+const createComment = async (feedId: number, body: CreateCommentRequest) => {
+  return api.post(END_POINT.FEED.COMMENTS(feedId), {
+    json: body,
+  });
+};
+
 export const COMMENT_QUERY_OPTIONS = {
   LIST: (feedId: number) =>
     queryOptions({
@@ -14,3 +23,16 @@ export const COMMENT_QUERY_OPTIONS = {
       queryFn: () => getComments(feedId),
     }),
 };
+
+export const COMMENT_MUTATION_OPTIONS = {
+  CREATE: () =>
+    mutationOptions({
+      mutationFn: ({
+        feedId,
+        body,
+      }: {
+        feedId: number;
+        body: CreateCommentRequest;
+      }) => createComment(feedId, body),
+    }),
+};
diff --git a/src/shared/api/end-point.ts b/src/shared/api/end-point.ts
index 8150945..05dbba9 100644
--- a/src/shared/api/end-point.ts
+++ b/src/shared/api/end-point.ts
@@ -3,7 +3,8 @@ export const END_POINT = {
     LIST: 'api/feeds',
     DETAIL: (feedId: number) => `api/feeds/${feedId}`,
     PARTICIPATION: (feedId: number) => `api/feeds/${feedId}/participations`,
-    COMMENTS: (feedId: number) => `api/feeds/${feedId}/comments`, // ✅ 추가
+    COMMENTS: (feedId: number) => `api/feeds/${feedId}/comments`,
+    CREATE: (feedId: number) => `api/feeds/${feedId}/comments`,
   },
   PARTICIPATION: {
     APPROVE: (id: number) => `api/participations/${id}/approve`,
diff --git a/src/shared/types/comments/type.ts b/src/shared/types/comments/type.ts
index 07858eb..6bad85c 100644
--- a/src/shared/types/comments/type.ts
+++ b/src/shared/types/comments/type.ts
@@ -2,3 +2,6 @@ import type { paths } from '@shared/types/schema';
 
 export type GetCommentsResponse =
   paths['/api/feeds/{feedId}/comments']['get']['responses']['200']['content']['*/*'];
+
+export type CreateCommentRequest =
+  paths['/api/feeds/{feedId}/comments']['post']['requestBody']['content']['application/json'];
diff --git a/src/widgets/postDetail/comment/comment-item.tsx b/src/widgets/postDetail/comment/comment-item.tsx
index e59a74d..43b0fa1 100644
--- a/src/widgets/postDetail/comment/comment-item.tsx
+++ b/src/widgets/postDetail/comment/comment-item.tsx
@@ -11,6 +11,7 @@ export interface CommentItemProps {
   memberId?: number;
   participationId?: number;
   createdAt?: string;
+  children?: CommentItemProps[];
 }
 
 export interface Participant {
@@ -21,6 +22,7 @@ export interface Participant {
 interface CommentItemUIProps extends CommentItemProps {
   isOwner?: boolean;
   participants?: Participant[];
+  onReply?: (commentId: number, nickname?: string) => void;
   onChangeApproval?: (
     participationId: number,
     status: Exclude<ApprovalStatus, 'pending'>,
@@ -32,7 +34,11 @@ export function CommentItem({
   description,
   commentType = 'USER',
   isOwner = false,
+  depth,
   onChangeApproval,
+  onReply,
+  commentId,
+  children,
   participationId,
   participants,
 }: CommentItemUIProps) {
@@ -77,10 +83,13 @@ export function CommentItem({
         <div className="mt-[0.4rem]">
           <p className="typo-body1 whitespace-pre-line">{description}</p>
 
-          {!isSystem && (
-            <button className="mt-[0.6rem] flex gap-[0.4rem] items-center typo-caption text-gray-500">
+          {depth === 0 && (
+            <button
+              onClick={() => onReply?.(commentId!, nickname)}
+              className="mt-[0.6rem] flex gap-[0.4rem] items-center typo-caption text-gray-500"
+            >
               <MessageIcon width={'1.8rem'} height={'1.8rem'} />
-              답글 쓰기
+              {children?.length ?? 0} 답글 쓰기
             </button>
           )}
         </div>
diff --git a/src/widgets/postDetail/comment/comment.tsx b/src/widgets/postDetail/comment/comment.tsx
index a889a74..de9bfb2 100644
--- a/src/widgets/postDetail/comment/comment.tsx
+++ b/src/widgets/postDetail/comment/comment.tsx
@@ -9,6 +9,7 @@ interface CommentProps {
   comments: CommentItemProps[];
   participants?: Participant[];
   isOwner?: boolean;
+  onReply?: (commentId: number, nickname?: string) => void; // 추가
   onChangeApproval?: (
     participationId: number,
     status: Exclude<ApprovalStatus, 'pending'>,
@@ -19,6 +20,7 @@ export function Comment({
   comments,
   participants,
   isOwner,
+  onReply,
   onChangeApproval,
 }: CommentProps) {
   const systemRoot = comments.filter(
@@ -40,23 +42,24 @@ export function Comment({
             {...comment}
             participants={participants}
             isOwner={isOwner}
+            onReply={onReply}
             onChangeApproval={onChangeApproval}
           />
         ))}
 
         {userRoot.map((comment) => (
-          <div className="flex flex-col gap-[2rem]" key={comment.commentId}>
+          <div className="flex flex-col" key={comment.commentId}>
             <CommentItem
               {...comment}
               participants={participants}
               isOwner={isOwner}
+              onReply={onReply}
               onChangeApproval={onChangeApproval}
             />
 
-            <div className="flex flex-col gap-[1rem] pl-[4.4rem]">
-              {comments
-                .filter((c) => c.parentId === comment.commentId)
-                .map((reply) => (
+            {comment.children && comment.children.length > 0 && (
+              <div className="flex flex-col gap-[1rem] pl-[4.4rem] pt-[2rem]">
+                {comment.children.map((reply) => (
                   <CommentItem
                     key={reply.commentId}
                     {...reply}
@@ -65,7 +68,8 @@ export function Comment({
                     onChangeApproval={onChangeApproval}
                   />
                 ))}
-            </div>
+              </div>
+            )}
           </div>
         ))}
       </div>