diff --git a/helpers/mock-agent/syndicator-mastodon.js b/helpers/mock-agent/syndicator-mastodon.js
index 9e71ee2f7..0cb1d7f4d 100644
--- a/helpers/mock-agent/syndicator-mastodon.js
+++ b/helpers/mock-agent/syndicator-mastodon.js
@@ -46,6 +46,48 @@ export function mockClient() {
     .intercept({ path: `/api/v1/statuses/404/reblog`, method: "POST" })
     .reply(404, { error: "Record not found" }, responseOptions);
 
+  // Search for a status on another server (found)
+  agent
+    .get(instanceOrigin)
+    .intercept({
+      path: (path) =>
+        path.startsWith("/api/v2/search?") &&
+        path.includes(encodeURIComponent("https://deadbird.example/")),
+    })
+    .reply(
+      200,
+      { accounts: [], hashtags: [], statuses: [{ id: "111" }] },
+      responseOptions,
+    )
+    .persist();
+
+  // Search for a status on another server (not found)
+  agent
+    .get(instanceOrigin)
+    .intercept({
+      path: (path) =>
+        path.startsWith("/api/v2/search?") &&
+        path.includes(encodeURIComponent("https://unknown.example/")),
+    })
+    .reply(200, { accounts: [], hashtags: [], statuses: [] }, responseOptions)
+    .persist();
+
+  // Post status in reply to a status on another server
+  agent
+    .get(instanceOrigin)
+    .intercept({
+      path: `/api/v1/statuses`,
+      headers: { authorization: "Bearer token" },
+      method: "POST",
+      body: (body) => body.includes('"in_reply_to_id":"111"'),
+    })
+    .reply(
+      200,
+      { id: "222", url: `https://mastodon.example/@username/reply` },
+      responseOptions,
+    )
+    .persist();
+
   // Post status
   agent
     .get(instanceOrigin)
diff --git a/packages/syndicator-mastodon/lib/mastodon.js b/packages/syndicator-mastodon/lib/mastodon.js
index 3ba5460c4..82892596f 100644
--- a/packages/syndicator-mastodon/lib/mastodon.js
+++ b/packages/syndicator-mastodon/lib/mastodon.js
@@ -33,6 +33,27 @@ export class Mastodon {
     });
   }
 
+  /**
+   * Resolve the URL of a status on another server to a status ID on this one
+   *
+   * Searching with `resolve` asks the server to fetch the status over
+   * ActivityPub if it hasn’t seen it before.
+   * @param {string} statusUrl - URL of status on another server
+   * @returns {Promise<string|undefined>} Status ID, if resolved
+   * @see {@link https://docs.joinmastodon.org/methods/search/}
+   */
+  async resolveRemoteStatus(statusUrl) {
+    const { v2 } = this.#client();
+    const { statuses } = await v2.search.list({
+      q: statusUrl,
+      type: "statuses",
+      resolve: true,
+      limit: 1,
+    });
+
+    return statuses[0]?.id;
+  }
+
   /**
    * Post a favourite
    * @param {string} statusUrl - URL of status to favourite
@@ -166,6 +187,15 @@ export class Mastodon {
       serverUrl: this.serverUrl,
     });
 
+    // Thread reply to a status on another server, if it can be resolved
+    const inReplyTo = properties["in-reply-to"];
+    if (status && inReplyTo && !isSameOrigin(inReplyTo, this.serverUrl)) {
+      const statusId = await this.resolveRemoteStatus(inReplyTo);
+      if (statusId) {
+        status.inReplyToId = statusId;
+      }
+    }
+
     if (status) {
       return this.postStatus(status);
     }
diff --git a/packages/syndicator-mastodon/lib/utils.js b/packages/syndicator-mastodon/lib/utils.js
index c593b0f0e..1a7a4a8d1 100644
--- a/packages/syndicator-mastodon/lib/utils.js
+++ b/packages/syndicator-mastodon/lib/utils.js
@@ -1,6 +1,5 @@
 import path from "node:path";
 
-import { IndiekitError } from "@indiekit/error";
 import brevity from "brevity";
 import { htmlToText } from "html-to-text";
 
@@ -79,18 +78,15 @@ export const createStatus = (properties, options = {}) => {
     parameters.mediaIds = mediaIds;
   }
 
-  // If post is in reply to a status, add respective parameter
+  // If post is in reply to a status on this server, add respective parameter.
+  // Replies to statuses on other servers are resolved before posting.
   if (properties["in-reply-to"]) {
     const inReplyTo = properties["in-reply-to"];
     const inReplyToHostname = new URL(inReplyTo).hostname;
     const serverHostname = new URL(serverUrl).hostname;
 
     if (inReplyToHostname === serverHostname) {
-      // Reply to status
-      const statusId = getStatusIdFromUrl(inReplyTo);
-      parameters.inReplyToId = statusId;
-    } else {
-      throw IndiekitError.badRequest("Not a reply to a URL at this target");
+      parameters.inReplyToId = getStatusIdFromUrl(inReplyTo);
     }
   }
 
diff --git a/packages/syndicator-mastodon/test/unit/mastodon.js b/packages/syndicator-mastodon/test/unit/mastodon.js
index 88018d9f4..3083bc042 100644
--- a/packages/syndicator-mastodon/test/unit/mastodon.js
+++ b/packages/syndicator-mastodon/test/unit/mastodon.js
@@ -145,6 +145,34 @@ describe("syndicator-mastodon/lib/mastodon", () => {
     );
   });
 
+  it("Posts a threaded reply to a status on another server", async () => {
+    const result = await mastodon.post(
+      {
+        content: {
+          html: "<p>I ate a cheese sandwich too!</p>",
+        },
+        "in-reply-to": "https://deadbird.example/username/1234567890987654321",
+      },
+      me,
+    );
+
+    assert.equal(result, "https://mastodon.example/@username/reply");
+  });
+
+  it("Posts an unthreaded reply if status on another server can’t be resolved", async () => {
+    const result = await mastodon.post(
+      {
+        content: {
+          html: "<p>I ate a cheese sandwich too!</p>",
+        },
+        "in-reply-to": "https://unknown.example/post",
+      },
+      me,
+    );
+
+    assert.equal(result, statusUrl);
+  });
+
   it("Posts a status with photo to Mastodon", async () => {
     const result = await mastodon.post(
       {
diff --git a/packages/syndicator-mastodon/test/unit/utils.js b/packages/syndicator-mastodon/test/unit/utils.js
index 264db3a19..5d7c20e1f 100644
--- a/packages/syndicator-mastodon/test/unit/utils.js
+++ b/packages/syndicator-mastodon/test/unit/utils.js
@@ -101,18 +101,16 @@ describe("syndicator-mastodon/lib/utils", () => {
     assert.equal(result.inReplyToId, "1234567890987654321");
   });
 
-  it("Throws creating a status if post is off-service reply", () => {
-    assert.throws(
-      () => {
-        createStatus(JSON.parse(getFixture("jf2/reply-off-service.jf2")), {
-          serverUrl: "https://mastodon.example",
-        });
-      },
+  it("Creates a status for a reply to a URL on another server", () => {
+    const result = createStatus(
+      JSON.parse(getFixture("jf2/reply-off-service.jf2")),
       {
-        name: "BadRequestError",
-        message: "Not a reply to a URL at this target",
+        serverUrl: "https://mastodon.example",
       },
     );
+
+    assert.equal(result.status, "I ate a cheese sandwich too!");
+    assert.equal(result.inReplyToId, undefined);
   });
 
   it("Creates a status with a photo", () => {