diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ed4bc77..8437ee2 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -64,6 +64,11 @@ jobs:
     - name: Build cpp example
       run: |
         just cmake_source_dir=ccp/example cmake_build_dir=cpp/example/build build-cpp
+    - name: Test cpp as external project
+      run: |
+        cd cpp/test/external_project
+        cmake -GNinja -S . -B build/
+        ninja -C build
     - id: prep
       name: Prepare deployment
       run: |
diff --git a/cpp/test/external_project/.gitignore b/cpp/test/external_project/.gitignore
new file mode 100644
index 0000000..567609b
--- /dev/null
+++ b/cpp/test/external_project/.gitignore
@@ -0,0 +1 @@
+build/
diff --git a/cpp/test/external_project/CMakeLists.txt b/cpp/test/external_project/CMakeLists.txt
new file mode 100644
index 0000000..43b4283
--- /dev/null
+++ b/cpp/test/external_project/CMakeLists.txt
@@ -0,0 +1,21 @@
+#  Copyright (C) 2026 University College London
+#  SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.12.0)
+cmake_policy(SET CMP0074 NEW) # find_package() uses <PackageName>_ROOT variables
+
+project(PETSIRDtest VERSION 0.1 LANGUAGES CXX)
+
+find_program(CCACHE_PROGRAM ccache)
+if(CCACHE_PROGRAM)
+  set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE "${CCACHE_PROGRAM}")
+  message(STATUS "ccache found, so we will use it.")
+endif()
+
+find_package(PETSIRD CONFIG REQUIRED)
+
+add_executable(minimal_test minimal_test.cpp)
+target_link_libraries(minimal_test PETSIRD::petsird)
+
+add_executable(minimal_test_helpers minimal_test_helpers.cpp)
+target_link_libraries(minimal_test_helpers   PETSIRD::petsird_helpers)
diff --git a/cpp/test/external_project/README.md b/cpp/test/external_project/README.md
new file mode 100644
index 0000000..8dadcd2
--- /dev/null
+++ b/cpp/test/external_project/README.md
@@ -0,0 +1,7 @@
+# Test files for using PETSIRD from C++ with CMake
+
+This directory contains test files for a minimal test if
+CMake can find PETSIRD with `find_package`, and if
+The C++ files compile and link.
+
+There is currently no run-time test.
diff --git a/cpp/test/external_project/minimal_test.cpp b/cpp/test/external_project/minimal_test.cpp
new file mode 100644
index 0000000..5e5f173
--- /dev/null
+++ b/cpp/test/external_project/minimal_test.cpp
@@ -0,0 +1,26 @@
+/*
+  Copyright (C) 2026 University College London
+
+  SPDX-License-Identifier: Apache-2.0
+*/
+
+// (un)comment if you want HDF5 or binary output
+#define USE_HDF5
+
+#ifdef USE_HDF5
+#  include "petsird/hdf5/protocols.h"
+using petsird::hdf5::PETSIRDReader;
+#else
+#  include "petsird/binary/protocols.h"
+using petsird::binary::PETSIRDReader;
+#endif
+
+int
+main(int, char const* argv[])
+{
+  PETSIRDReader reader(argv[1]);
+  petsird::Header header;
+  reader.ReadHeader(header);
+
+  return 0;
+}
diff --git a/cpp/test/external_project/minimal_test_helpers.cpp b/cpp/test/external_project/minimal_test_helpers.cpp
new file mode 100644
index 0000000..b9764dc
--- /dev/null
+++ b/cpp/test/external_project/minimal_test_helpers.cpp
@@ -0,0 +1,29 @@
+/*
+  Copyright (C) 2026 University College London
+
+  SPDX-License-Identifier: Apache-2.0
+*/
+
+// (un)comment if you want HDF5 or binary output
+#define USE_HDF5
+
+#ifdef USE_HDF5
+#  include "petsird/hdf5/protocols.h"
+using petsird::hdf5::PETSIRDReader;
+#else
+#  include "petsird/binary/protocols.h"
+using petsird::binary::PETSIRDReader;
+#endif
+#include "petsird_helpers.h"
+#include "petsird_helpers/geometry.h"
+#include "petsird_helpers/create.h"
+
+int
+main(int, char const* argv[])
+{
+  PETSIRDReader reader(argv[1]);
+  petsird::Header header;
+  reader.ReadHeader(header);
+
+  return 0;
+}