diff --git a/docs/notification.md b/docs/notification.md
index 6e70c86..6563bbf 100644
--- a/docs/notification.md
+++ b/docs/notification.md
@@ -1,6 +1,7 @@
 # Notification
 
 - [getItems](#getitems)
+- [addItem](#additem)
 
 ## Overview
 
@@ -40,3 +41,31 @@ See the example response in the `Reference`.
 #### Reference
 
 - https://developer.cybozu.io/hc/ja/articles/360017764051#step1
+
+### addItem
+
+Add a notification.
+
+#### Parameters
+
+| Name                |       Type       |          Required           | Description                                                                        |
+| ------------------- | :--------------: | :-------------------------: | ---------------------------------------------------------------------------------- |
+| app                 |      String      |             Yes             | The external notification code.                                                    |
+| notificationKey     |      String      |             Yes             | The notification key.                                                              |
+| operation           |      String      |             Yes             | The operation of the notification. Possible values are: `add`, `modify`, `remove`. |
+| url                 |      String      |             Yes             | The notification URL.                                                              |
+| title               |      String      |             Yes             | The notification title.                                                            |
+| body                |      String      |             Yes             | The notification body.                                                             |
+| icon                |      String      |                             | The notification icon URL.                                                         |
+| destinations        | Array\<Object\>  |             Yes             | The list of the notification destinations.                                         |
+| destinations[].type |      String      |             Yes             | The destination type. Possible value is: `USER`.                                   |
+| destinations[].id   | String or Number | Conditionally<br />Required | The ID of the destination. Required if `destinations[].code` is not specified.     |
+| destinations[].code |      String      | Conditionally<br />Required | The code of the destination. Required if `destinations[].id` is not specified.     |
+
+#### Returns
+
+See the example response in the `Reference`.
+
+#### Reference
+
+- https://developer.cybozu.io/hc/ja/articles/360017482672#step1
diff --git a/src/client/NotificationClient.ts b/src/client/NotificationClient.ts
index 893a469..0da9b45 100644
--- a/src/client/NotificationClient.ts
+++ b/src/client/NotificationClient.ts
@@ -43,4 +43,37 @@ export class NotificationClient {
     }
     return this.client.get(path, data);
   }
+
+  public addItem(params: {
+    app: string;
+    notificationKey: string;
+    operation: "add" | "modify" | "remove";
+    url: string;
+    title: string;
+    body: string;
+    icon?: string;
+    destinations: Array<{
+      type: "USER";
+      id?: string | number;
+      code?: string;
+    }>;
+  }): Promise<{
+    moduleId: string;
+    notificationKey: string;
+    creator: {
+      id: string;
+      code: string;
+      name: string;
+    };
+    createdAt: string;
+    operation: "add" | "modify" | "remove";
+    url: string;
+    title: string;
+    body: string;
+    icon: string;
+    isRead: boolean;
+  }> {
+    const path = buildPath({ endpointName: `notification/items` });
+    return this.client.post(path, params);
+  }
 }
diff --git a/src/client/__tests__/NotificationClient.test.ts b/src/client/__tests__/NotificationClient.test.ts
index 303d32d..fc44d09 100644
--- a/src/client/__tests__/NotificationClient.test.ts
+++ b/src/client/__tests__/NotificationClient.test.ts
@@ -43,4 +43,34 @@ describe("NotificationClient", () => {
       });
     });
   });
+
+  describe("addItem", () => {
+    const params = {
+      app: "Test",
+      notificationKey: "test-notification-key",
+      operation: "add" as const,
+      url: "http://example.com",
+      title: "Test title",
+      body: "Test body",
+      icon: "http://example.com/example.png",
+      destinations: [
+        {
+          type: "USER" as const,
+          id: "1",
+        },
+      ],
+    };
+    beforeEach(async () => {
+      await notificationClient.addItem(params);
+    });
+    it("should pass the path to the http client", () => {
+      expect(mockClient.getLogs()[0].path).toBe("/api/v1/notification/items");
+    });
+    it("should send a post request", () => {
+      expect(mockClient.getLogs()[0].method).toBe("post");
+    });
+    it("should pass params as a param to the http client", () => {
+      expect(mockClient.getLogs()[0].params).toEqual(params);
+    });
+  });
 });