diff --git a/Modules/FetchContent/CMakeLists.cmake.in b/Modules/FetchContent/CMakeLists.cmake.in
index aa2ad9918b..8adb533500 100644
--- a/Modules/FetchContent/CMakeLists.cmake.in
+++ b/Modules/FetchContent/CMakeLists.cmake.in
@@ -3,6 +3,12 @@
 
 cmake_minimum_required(VERSION @CMAKE_VERSION@)
 
+# Reject any attempt to use a toolchain file. We must not use one because
+# we could be downloading it here. If the CMAKE_TOOLCHAIN_FILE environment
+# variable is set, the cache variable will have been initialized from it.
+unset(CMAKE_TOOLCHAIN_FILE CACHE)
+unset(ENV{CMAKE_TOOLCHAIN_FILE})
+
 # We name the project and the target for the ExternalProject_Add() call
 # to something that will highlight to the user what we are working on if
 # something goes wrong and an error message is produced.
diff --git a/Tests/RunCMake/FetchContent/IgnoreToolchainFile.cmake b/Tests/RunCMake/FetchContent/IgnoreToolchainFile.cmake
new file mode 100644
index 0000000000..f8ee74995c
--- /dev/null
+++ b/Tests/RunCMake/FetchContent/IgnoreToolchainFile.cmake
@@ -0,0 +1,2 @@
+set(ENV{CMAKE_TOOLCHAIN_FILE} path/to/somewhere/iDoNotExist.cmake)
+include(DownloadFile.cmake)
diff --git a/Tests/RunCMake/FetchContent/RunCMakeTest.cmake b/Tests/RunCMake/FetchContent/RunCMakeTest.cmake
index bb27491b6b..378108991a 100644
--- a/Tests/RunCMake/FetchContent/RunCMakeTest.cmake
+++ b/Tests/RunCMake/FetchContent/RunCMakeTest.cmake
@@ -7,6 +7,7 @@ run_cmake(DirectIgnoresDetails)
 run_cmake(FirstDetailsWin)
 run_cmake(DownloadTwice)
 run_cmake(DownloadFile)
+run_cmake(IgnoreToolchainFile)
 run_cmake(SameGenerator)
 run_cmake(System)
 run_cmake(VarDefinitions)