diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a3664ddf55a..2348212f4b8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -144,6 +144,24 @@ jobs:
       if: "!(runner.os == 'Windows' && (matrix.python == 3.8 || matrix.python == 3.9 || matrix.python == '3.10-dev'))"
       run: cmake --build build2 --target cpptest
 
+    # Third build - C++17 mode with unstable ABI
+    - name: Configure (unstable ABI)
+      run: >
+        cmake -S . -B build3
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DDOWNLOAD_EIGEN=ON
+        -DCMAKE_CXX_STANDARD=17
+        -DPYBIND11_INTERNALS_VERSION=10000000
+        "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
+        ${{ matrix.args }}
+
+    - name: Build (unstable ABI)
+      run: cmake --build build3 -j 2
+
+    - name: Python tests (unstable ABI)
+      run: cmake --build build3 --target pytest
+
     - name: Interface test
       run: cmake --build build2 --target test_cmake_build
 
diff --git a/CMakeLists.txt b/CMakeLists.txt
index bb01a861aef..25cfcec2f6c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -89,6 +89,9 @@ endif()
 option(PYBIND11_INSTALL "Install pybind11 header files?" ${PYBIND11_MASTER_PROJECT})
 option(PYBIND11_TEST "Build pybind11 test suite?" ${PYBIND11_MASTER_PROJECT})
 option(PYBIND11_NOPYTHON "Disable search for Python" OFF)
+set(PYBIND11_INTERNALS_VERSION
+    ""
+    CACHE STRING "Override the ABI version, may be used to enable the unstable ABI.")
 
 cmake_dependent_option(
   USE_PYTHON_INCLUDE_DIR
@@ -183,6 +186,10 @@ if(NOT TARGET pybind11_headers)
 
   target_compile_features(pybind11_headers INTERFACE cxx_inheriting_constructors cxx_user_literals
                                                      cxx_right_angle_brackets)
+  if(NOT "${PYBIND11_INTERNALS_VERSION}" STREQUAL "")
+    target_compile_definitions(
+      pybind11_headers INTERFACE "PYBIND11_INTERNALS_VERSION=${PYBIND11_INTERNALS_VERSION}")
+  endif()
 else()
   # It is invalid to install a target twice, too.
   set(PYBIND11_INSTALL OFF)