diff --git a/src/backlog.ts b/src/backlog.ts
index ae5939b..2c723be 100644
--- a/src/backlog.ts
+++ b/src/backlog.ts
@@ -810,6 +810,39 @@ export default class Backlog extends Request {
     return this.delete(`issues/${issueIdOrKey}/sharedFiles/${id}`);
   }
 
+  /**
+   * Get the list of issues related to the specified issue.
+   *
+   * GET /api/v2/issues/:issueIdOrKey/relatedIssues
+   */
+  public getRelatedIssues(issueIdOrKey: string | number): Promise<Entity.Issue.RelatedIssue[]> {
+    return this.get(`issues/${issueIdOrKey}/relatedIssues`);
+  }
+
+  /**
+   * Add a related issue to the specified issue.
+   *
+   * POST /api/v2/issues/:issueIdOrKey/relatedIssues
+   */
+  public addRelatedIssue(
+    issueIdOrKey: string | number,
+    params: Option.Issue.AddRelatedIssueParams,
+  ): Promise<Entity.Issue.RelatedIssue> {
+    return this.post(`issues/${issueIdOrKey}/relatedIssues`, params);
+  }
+
+  /**
+   * Remove a related issue from the specified issue.
+   *
+   * DELETE /api/v2/issues/:issueIdOrKey/relatedIssues/:relatedIssueId
+   */
+  public removeRelatedIssue(
+    issueIdOrKey: string | number,
+    relatedIssueId: number,
+  ): Promise<Entity.Issue.RelatedIssue> {
+    return this.delete(`issues/${issueIdOrKey}/relatedIssues/${relatedIssueId}`);
+  }
+
   /**
    * https://developer.nulab.com/docs/backlog/api/2/get-wiki-page-list/
    */
diff --git a/src/entity.ts b/src/entity.ts
index 3d85daf..c5f2789 100644
--- a/src/entity.ts
+++ b/src/entity.ts
@@ -624,6 +624,10 @@ export namespace Issue {
     childIssueSummary?: ChildIssueSummary;
   }
 
+  export interface RelatedIssue extends Issue {
+    type: Types.IssueRelationType;
+  }
+
   export interface ChildIssueSummary {
     total: number;
     closed: number;
diff --git a/src/option.ts b/src/option.ts
index 1755c9d..02efb79 100644
--- a/src/option.ts
+++ b/src/option.ts
@@ -462,6 +462,10 @@ export namespace Issue {
   export interface LinkIssueSharedFilesParams {
     fileId: number[];
   }
+
+  export interface AddRelatedIssueParams {
+    targetIssueId: number;
+  }
 }
 
 export namespace PullRequest {
diff --git a/src/types.ts b/src/types.ts
index 8b36403..3eab08e 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -99,6 +99,9 @@ export enum CustomFieldType {
   Radio = 8,
 }
 
+/** Relation type of a related issue. Only the undirected `RELATES` link is available. */
+export type IssueRelationType = "RELATES";
+
 export type WebhookActivityId = number;
 
 export type Fetch = typeof globalThis.fetch;
diff --git a/test/test.ts b/test/test.ts
index 09e5c13..05fa3ce 100644
--- a/test/test.ts
+++ b/test/test.ts
@@ -306,6 +306,81 @@ describe("Backlog API", () => {
     expect(data.childIssueSummary).toEqual({ total: 3, closed: 1 });
   });
 
+  it("should get related issues.", async () => {
+    const relatedIssues = [
+      {
+        id: 2,
+        projectId: 1,
+        issueKey: "TEST-2",
+        keyId: 2,
+        summary: "related",
+        type: "RELATES",
+      },
+      {
+        id: 3,
+        projectId: 1,
+        issueKey: "TEST-3",
+        keyId: 3,
+        summary: "related too",
+        type: "RELATES",
+      },
+    ];
+    mockRequest({
+      method: "GET",
+      path: "/api/v2/issues/TEST-1/relatedIssues",
+      query: { apiKey },
+      status: 200,
+      data: relatedIssues,
+      times: 1,
+    });
+    const data = await backlog.getRelatedIssues("TEST-1");
+    expect(data).toEqual(relatedIssues);
+    expect(data[0].type).toBe("RELATES");
+  });
+
+  it("should add a related issue.", async () => {
+    const relatedIssue = {
+      id: 2,
+      projectId: 1,
+      issueKey: "TEST-2",
+      keyId: 2,
+      summary: "related",
+      type: "RELATES",
+    };
+    mockRequest({
+      method: "POST",
+      path: "/api/v2/issues/TEST-1/relatedIssues",
+      query: { apiKey },
+      body: qs.stringify({ targetIssueId: 2 }),
+      status: 200,
+      data: relatedIssue,
+      times: 1,
+    });
+    const data = await backlog.addRelatedIssue("TEST-1", { targetIssueId: 2 });
+    expect(data).toEqual(relatedIssue);
+  });
+
+  it("should remove a related issue.", async () => {
+    const relatedIssue = {
+      id: 2,
+      projectId: 1,
+      issueKey: "TEST-2",
+      keyId: 2,
+      summary: "related",
+      type: "RELATES",
+    };
+    mockRequest({
+      method: "DELETE",
+      path: "/api/v2/issues/TEST-1/relatedIssues/2",
+      query: { apiKey },
+      status: 200,
+      data: relatedIssue,
+      times: 1,
+    });
+    const data = await backlog.removeRelatedIssue("TEST-1", 2);
+    expect(data).toEqual(relatedIssue);
+  });
+
   it("should serialize the expand parameter using bracket array format.", () => {
     const query = (backlog as any).toQueryString({
       parentChild: backlogjs.Option.Issue.ParentChildType.GrandchildOnly,