diff --git a/repos/spack_repo/builtin/packages/vtk_m/package.py b/repos/spack_repo/builtin/packages/vtk_m/package.py
index 5601a415632..07f8a6109aa 100644
--- a/repos/spack_repo/builtin/packages/vtk_m/package.py
+++ b/repos/spack_repo/builtin/packages/vtk_m/package.py
@@ -107,6 +107,7 @@ class VtkM(CMakePackage, CudaPackage, ROCmPackage):
     # VTK-m uses the default Kokkos backend
     depends_on("kokkos", when="+kokkos")
     depends_on("kokkos@3.7:3.9", when="@2.0 +kokkos")
+    conflicts("^kokkos@5:", when="+kokkos", msg="vtk-m doesn't compile with C++20")
     # VTK-m native CUDA and Kokkos CUDA backends are not compatible
     depends_on("kokkos ~cuda", when="+kokkos +cuda +cuda_native")
     depends_on("kokkos +cuda", when="+kokkos +cuda ~cuda_native")