diff --git a/.changeset/socket-mode-undici-v8.md b/.changeset/socket-mode-undici-v8.md
new file mode 100644
index 000000000..d2ff20232
--- /dev/null
+++ b/.changeset/socket-mode-undici-v8.md
@@ -0,0 +1,5 @@
+---
+"@slack/socket-mode": minor
+---
+
+Support `undici@^8` as a peer dependency in addition to `undici@^7`. `undici@^8` requires Node.js >=22.19; `undici@^7` continues to support Node.js >=20.
diff --git a/.github/workflows/ci-build.yml b/.github/workflows/ci-build.yml
index d5afe57af..fffb8dd41 100644
--- a/.github/workflows/ci-build.yml
+++ b/.github/workflows/ci-build.yml
@@ -18,14 +18,20 @@ jobs:
         os:
           - "ubuntu-latest"
           - "windows-latest"
-        node-version:
-          - "20.x"
-          - "22.x"
-          - "24.x"
-          - "26.x"
+        versions:
+          - node: "20.x"
+            undici: "7" # undici v8 requires Node >=22.19, so 20.x tests against v7
+          - node: "22.x"
+            undici: "8"
+          - node: "24.x"
+            undici: "8"
+          - node: "26.x"
+            undici: "8"
     runs-on: ${{ matrix.os }}
     permissions:
       contents: read
+    env:
+      SOCKET_MODE_UNDICI_VERSION: ${{ matrix.versions.undici }}
     steps:
       - name: Configure git settings (Windows)
         if: matrix.os == 'windows-latest'
@@ -35,31 +41,34 @@ jobs:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
-      - name: Use Node.js ${{ matrix.node-version }}
+      - name: Use Node.js ${{ matrix.versions.node }}
         uses: actions/setup-node@820762786026740c76f36085b0efc47a31fe5020 # v7.0.0
         with:
-          node-version: ${{ matrix.node-version }}
+          node-version: ${{ matrix.versions.node }}
       - name: Check versions
         run: |
           node --version
           npm --version
       - name: Install dependencies
         run: npm ci --verbose
+      # Force socket-mode onto the matrix's undici major.
+      - name: Install undici v${{ matrix.versions.undici }} for socket-mode
+        run: npm install undici@^${{ matrix.versions.undici }} --workspace=packages/socket-mode
       - name: Build packages
         run: npm run build
       - name: Lint
         run: npm run lint
       - name: Build docs
-        if: matrix.node-version == env.LATEST_SUPPORTED_NODE
+        if: matrix.versions.node == env.LATEST_SUPPORTED_NODE
         run: npm run docs
       - name: Run tests
-        if: matrix.node-version != env.LATEST_SUPPORTED_NODE
+        if: matrix.versions.node != env.LATEST_SUPPORTED_NODE
         run: npm test
       - name: Run test coverage
-        if: matrix.node-version == env.LATEST_SUPPORTED_NODE
+        if: matrix.versions.node == env.LATEST_SUPPORTED_NODE
         run: npm run test:coverage
       - name: Upload code coverage
-        if: matrix.node-version == env.LATEST_SUPPORTED_NODE && matrix.os == 'ubuntu-latest'
+        if: matrix.versions.node == env.LATEST_SUPPORTED_NODE && matrix.os == 'ubuntu-latest'
         uses: codecov/codecov-action@fb8b3582c8e4def4969c97caa2f19720cb33a72f # v7.0.0
         with:
           fail_ci_if_error: true
@@ -74,7 +83,7 @@ jobs:
         with:
           fail_ci_if_error: true
           files: packages/cli-hooks/test-results.xml,packages/cli-test/test-results.xml,packages/logger/test-results.xml,packages/oauth/test-results.xml,packages/socket-mode/test-results.xml,packages/web-api/test-results.xml,packages/webhook/test-results.xml
-          flags: ${{ matrix.node-version }},${{ matrix.os }}
+          flags: ${{ matrix.versions.node }},${{ matrix.os }}
           report_type: test_results
           token: ${{ secrets.CODECOV_TOKEN }}
           verbose: true
diff --git a/packages/socket-mode/package.json b/packages/socket-mode/package.json
index 43222aad7..0d606f580 100644
--- a/packages/socket-mode/package.json
+++ b/packages/socket-mode/package.json
@@ -57,7 +57,7 @@
     "eventemitter3": "^5"
   },
   "peerDependencies": {
-    "undici": "^7.0.0"
+    "undici": "^7.0.0 || ^8.0.0"
   },
   "devDependencies": {
     "@types/proxyquire": "^1.3.31",
diff --git a/packages/socket-mode/src/undiciVersion.test.ts b/packages/socket-mode/src/undiciVersion.test.ts
new file mode 100644
index 000000000..5ea57b350
--- /dev/null
+++ b/packages/socket-mode/src/undiciVersion.test.ts
@@ -0,0 +1,33 @@
+import assert from 'node:assert/strict';
+import { createRequire } from 'node:module';
+import { describe, it } from 'node:test';
+
+// Guardrail for the `undici` peer dependency. undici v8 needs
+// Node >=22 and v7 needs Node >=20, so CI passes the expected major in via SOCKET_MODE_UNDICI_VERSION.
+
+const require = createRequire(import.meta.url);
+const undiciVersion: string = require('undici/package.json').version;
+const undiciMajor = Number.parseInt(undiciVersion.split('.')[0], 10);
+const nodeMajor = Number.parseInt(process.versions.node.split('.')[0], 10);
+
+describe('undici peer dependency', () => {
+  it('never runs undici v8+ on Node older than 22', () => {
+    assert.ok(
+      undiciMajor < 8 || nodeMajor >= 22,
+      `undici@${undiciVersion} requires Node >=22, but tests are running on Node ${process.versions.node}`,
+    );
+  });
+
+  const expectedMajor = process.env.SOCKET_MODE_UNDICI_VERSION;
+  if (expectedMajor) {
+    it(`runs against the undici major pinned by CI (v${expectedMajor})`, () => {
+      assert.strictEqual(
+        undiciMajor,
+        Number.parseInt(expectedMajor, 10),
+        `Expected undici v${expectedMajor} (SOCKET_MODE_UNDICI_VERSION), but undici@${undiciVersion} is installed`,
+      );
+    });
+  } else {
+    it.skip('runs against the undici major pinned by CI (only when SOCKET_MODE_UNDICI_VERSION is set)');
+  }
+});