diff --git a/src/main/resources/openapi/api.yaml b/src/main/resources/openapi/api.yaml
index 3cc79aab..e07dec94 100644
--- a/src/main/resources/openapi/api.yaml
+++ b/src/main/resources/openapi/api.yaml
@@ -74,6 +74,71 @@ paths:
           $ref: "#/components/responses/NotFoundOrForbidden"
         429:
           $ref: "#/components/responses/TooManyRequests"
+  /posts/{postId}/reactions:
+    get:
+      summary: リアクションを数件返す
+      security:
+        - BearerAuth: [ ]
+      parameters:
+        - $ref: "#/components/parameters/postId"
+      responses:
+        200:
+          description: 成功
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ReactionResponse"
+        401:
+          $ref: "#/components/responses/Unauthorized"
+        403:
+          $ref: "#/components/responses/Forbidden"
+        404:
+          $ref: "#/components/responses/NotFoundOrForbidden"
+        429:
+          $ref: "#/components/responses/TooManyRequests"
+
+    post:
+      summary: リアクションを付ける
+      security:
+        - BearerAuth: [ ]
+      parameters:
+        - $ref: "#/components/parameters/postId"
+      requestBody:
+        description: 付けるリアクション
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/Reaction"
+      responses:
+        200:
+          description: 成功
+        401:
+          $ref: "#/components/responses/Unauthorized"
+        403:
+          $ref: "#/components/responses/Forbidden"
+        404:
+          $ref: "#/components/responses/NotFoundOrForbidden"
+        429:
+          $ref: "#/components/responses/TooManyRequests"
+    delete:
+      summary: リアクションを削除する
+      security:
+        - BearerAuth: [ ]
+      parameters:
+        - $ref: "#/components/parameters/postId"
+      responses:
+        200:
+          description: 成功
+        401:
+          $ref: "#/components/responses/Unauthorized"
+        403:
+          $ref: "#/components/responses/Forbidden"
+        404:
+          $ref: "#/components/responses/NotFoundOrForbidden"
+        429:
+          $ref: "#/components/responses/TooManyRequests"
+
   /users/{userName}/posts:
     get:
       summary: 権限に応じてユーザーの投稿一覧を返す
@@ -421,6 +486,36 @@ components:
         password:
           type: string
 
+    Reaction:
+      type: object
+      properties:
+        reaction:
+          type: string
+
+    ReactionResponse:
+      type: object
+      properties:
+        reaction:
+          type: string
+        isUnicodeEmoji:
+          type: boolean
+        iconUrl:
+          type: string
+        accounts:
+          type: array
+          items:
+            $ref: "#/components/schemas/Account"
+
+    Account:
+      type: object
+      properties:
+        screenName:
+          type: string
+        iconUrl:
+          type: string
+        url:
+          type: string
+
   securitySchemes:
     BearerAuth:
       type: http