diff --git a/CMakeLists.txt b/CMakeLists.txt
index a5835bd24e..33c9a30e35 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -8,6 +8,8 @@
 
 cmake_minimum_required(VERSION 3.20)
 
+# Set minimum policy version to 3.5 to fix FFTW compilation with CMake 4.0+
+set(CMAKE_POLICY_VERSION_MINIMUM "3.5" CACHE STRING "Minimum CMake policy version")
 
 # We include C as a language because - for some reason -
 # FIND_LIBRARY_USE_LIB64_PATHS is otherwise ignored.
diff --git a/toolchain/dependencies/CMakeLists.txt b/toolchain/dependencies/CMakeLists.txt
index 116e288711..610be1f858 100644
--- a/toolchain/dependencies/CMakeLists.txt
+++ b/toolchain/dependencies/CMakeLists.txt
@@ -36,6 +36,7 @@ if (MFC_FFTW)
                 URL        "http://www.fftw.org/fftw-3.3.10.tar.gz"
                 CMAKE_ARGS -DBUILD_TESTS=OFF
                            -DBUILD_SHARED_LIBS=OFF
+                           -DCMAKE_POLICY_VERSION_MINIMUM=3.5
                            "-DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}"
             )
         else()