diff --git a/Help/command/project.rst b/Help/command/project.rst
index 2a9dcfe..8f32fa3 100644
--- a/Help/command/project.rst
+++ b/Help/command/project.rst
@@ -123,28 +123,56 @@
 The variables set through the ``VERSION``, ``DESCRIPTION`` and ``HOMEPAGE_URL``
 options are intended for use as default values in package metadata and documentation.
 
+.. _`Code Injection`:
+
 Code Injection
 ^^^^^^^^^^^^^^
 
-If the :variable:`CMAKE_PROJECT_INCLUDE_BEFORE` or
-:variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE_BEFORE` variables are set,
-the files they point to will be included as the first step of the
-``project()`` command.
-If both are set, then :variable:`CMAKE_PROJECT_INCLUDE_BEFORE` will be
-included before :variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE_BEFORE`.
+A number of variables can be defined by the user to specify files to include
+at different points during the execution of the ``project()`` command.
+The following outlines the steps performed during a ``project()`` call:
 
-If the :variable:`CMAKE_PROJECT_INCLUDE` or
-:variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE` variables are set, the files
-they point to will be included as the last step of the ``project()`` command.
-If both are set, then :variable:`CMAKE_PROJECT_INCLUDE` will be included before
-:variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE`.
+* .. versionadded:: 3.15
+    For every ``project()`` call regardless of the project
+    name, include the file named by :variable:`CMAKE_PROJECT_INCLUDE_BEFORE`,
+    if set.
 
-.. versionadded:: 3.15
-  Added the ``CMAKE_PROJECT_INCLUDE`` and ``CMAKE_PROJECT_INCLUDE_BEFORE``
-  variables.
+* .. versionadded:: 3.17
+    If the ``project()`` command specifies ``<PROJECT-NAME>`` as its project
+    name, include the file named by
+    :variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE_BEFORE`, if set.
 
-.. versionadded:: 3.17
-  Added the ``CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE_BEFORE`` variable.
+* Set the various project-specific variables detailed in the `Synopsis`_
+  and `Options`_ sections above.
+
+* For the very first ``project()`` call only:
+
+  * If :variable:`CMAKE_TOOLCHAIN_FILE` is set, read it at least once.
+    It may be read multiple times and it may also be read again when
+    enabling languages later (see below).
+
+  * Set the variables describing the host and target platforms.
+    Language-specific variables might or might not be set at this point.
+    On the first run, the only language-specific variables that might be
+    defined are those a toolchain file may have set. On subsequent runs,
+    language-specific variables cached from a previous run may be set.
+
+  * .. versionadded:: 3.24
+      Include each file listed in :variable:`CMAKE_PROJECT_TOP_LEVEL_INCLUDES`,
+      if set. The variable is ignored by CMake thereafter.
+
+* Enable any languages specified in the call, or the default languages if
+  none were provided. The toolchain file may be re-read when enabling a
+  language for the first time.
+
+* .. versionadded:: 3.15
+    For every ``project()`` call regardless of the project
+    name, include the file named by :variable:`CMAKE_PROJECT_INCLUDE`,
+    if set.
+
+* If the ``project()`` command specifies ``<PROJECT-NAME>`` as its project
+  name, include the file named by
+  :variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE`, if set.
 
 Usage
 ^^^^^
diff --git a/Help/manual/cmake-variables.7.rst b/Help/manual/cmake-variables.7.rst
index 80160b6..00ea0bc 100644
--- a/Help/manual/cmake-variables.7.rst
+++ b/Help/manual/cmake-variables.7.rst
@@ -243,6 +243,7 @@
    /variable/CMAKE_PROJECT_INCLUDE_BEFORE
    /variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE
    /variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE_BEFORE
+   /variable/CMAKE_PROJECT_TOP_LEVEL_INCLUDES
    /variable/CMAKE_REQUIRE_FIND_PACKAGE_PackageName
    /variable/CMAKE_SKIP_INSTALL_ALL_DEPENDENCY
    /variable/CMAKE_STAGING_PREFIX
diff --git a/Help/release/dev/CMAKE_PROJECT_TOP_LEVEL_INCLUDES.rst b/Help/release/dev/CMAKE_PROJECT_TOP_LEVEL_INCLUDES.rst
new file mode 100644
index 0000000..ca2d223
--- /dev/null
+++ b/Help/release/dev/CMAKE_PROJECT_TOP_LEVEL_INCLUDES.rst
@@ -0,0 +1,6 @@
+CMAKE_PROJECT_TOP_LEVEL_INCLUDES
+--------------------------------
+
+* The :variable:`CMAKE_PROJECT_TOP_LEVEL_INCLUDES` variable was added to allow
+  injecting custom code at the site of the first :command:`project` call,
+  after the host and target platform details have been determined.
diff --git a/Help/variable/CMAKE_PROJECT_INCLUDE.rst b/Help/variable/CMAKE_PROJECT_INCLUDE.rst
index 41d9e5d..76b9d92 100644
--- a/Help/variable/CMAKE_PROJECT_INCLUDE.rst
+++ b/Help/variable/CMAKE_PROJECT_INCLUDE.rst
@@ -5,8 +5,11 @@
 
 A CMake language file or module to be included as the last step of all
 :command:`project` command calls.  This is intended for injecting custom code
-into project builds without modifying their source.
+into project builds without modifying their source.  See :ref:`Code Injection`
+for a more detailed discussion of files potentially included during a
+:command:`project` call.
 
 See also the :variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE`,
-:variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE_BEFORE` and
-:variable:`CMAKE_PROJECT_INCLUDE_BEFORE` variables.
+:variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE_BEFORE`,
+:variable:`CMAKE_PROJECT_INCLUDE_BEFORE`, and
+:variable:`CMAKE_PROJECT_TOP_LEVEL_INCLUDES` variables.
diff --git a/Help/variable/CMAKE_PROJECT_INCLUDE_BEFORE.rst b/Help/variable/CMAKE_PROJECT_INCLUDE_BEFORE.rst
index c2fd0f8..9a8c4b5 100644
--- a/Help/variable/CMAKE_PROJECT_INCLUDE_BEFORE.rst
+++ b/Help/variable/CMAKE_PROJECT_INCLUDE_BEFORE.rst
@@ -5,8 +5,11 @@
 
 A CMake language file or module to be included as the first step of all
 :command:`project` command calls.  This is intended for injecting custom code
-into project builds without modifying their source.
+into project builds without modifying their source.  See :ref:`Code Injection`
+for a more detailed discussion of files potentially included during a
+:command:`project` call.
 
 See also the :variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE`,
-:variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE_BEFORE` and
-:variable:`CMAKE_PROJECT_INCLUDE` variables.
+:variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE_BEFORE`,
+:variable:`CMAKE_PROJECT_INCLUDE`, and
+:variable:`CMAKE_PROJECT_TOP_LEVEL_INCLUDES` variables.
diff --git a/Help/variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE.rst b/Help/variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE.rst
index 74247f1..3bb5cd8 100644
--- a/Help/variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE.rst
+++ b/Help/variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE.rst
@@ -4,8 +4,9 @@
 A CMake language file or module to be included as the last step of any
 :command:`project` command calls that specify ``<PROJECT-NAME>`` as the project
 name.  This is intended for injecting custom code into project builds without
-modifying their source.
+modifying their source.  See :ref:`Code Injection` for a more detailed
+discussion of files potentially included during a :command:`project` call.
 
 See also the :variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE_BEFORE`,
-:variable:`CMAKE_PROJECT_INCLUDE` and
-:variable:`CMAKE_PROJECT_INCLUDE_BEFORE` variables.
+:variable:`CMAKE_PROJECT_INCLUDE`, :variable:`CMAKE_PROJECT_INCLUDE_BEFORE`,
+and :variable:`CMAKE_PROJECT_TOP_LEVEL_INCLUDES` variables.
diff --git a/Help/variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE_BEFORE.rst b/Help/variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE_BEFORE.rst
index 39abb12..ca584c1 100644
--- a/Help/variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE_BEFORE.rst
+++ b/Help/variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE_BEFORE.rst
@@ -6,8 +6,9 @@
 A CMake language file or module to be included as the first step of any
 :command:`project` command calls that specify ``<PROJECT-NAME>`` as the project
 name.  This is intended for injecting custom code into project builds without
-modifying their source.
+modifying their source.  See :ref:`Code Injection` for a more detailed
+discussion of files potentially included during a :command:`project` call.
 
 See also the :variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE`,
-:variable:`CMAKE_PROJECT_INCLUDE` and
-:variable:`CMAKE_PROJECT_INCLUDE_BEFORE` variables.
+:variable:`CMAKE_PROJECT_INCLUDE`, :variable:`CMAKE_PROJECT_INCLUDE_BEFORE`,
+and :variable:`CMAKE_PROJECT_TOP_LEVEL_INCLUDES` variables.
diff --git a/Help/variable/CMAKE_PROJECT_TOP_LEVEL_INCLUDES.rst b/Help/variable/CMAKE_PROJECT_TOP_LEVEL_INCLUDES.rst
new file mode 100644
index 0000000..2010b08
--- /dev/null
+++ b/Help/variable/CMAKE_PROJECT_TOP_LEVEL_INCLUDES.rst
@@ -0,0 +1,27 @@
+CMAKE_PROJECT_TOP_LEVEL_INCLUDES
+--------------------------------
+
+.. versionadded:: 3.24
+
+:ref:`Semicolon-separated list <CMake Language Lists>` of CMake language
+files to include as part of the very first :command:`project` call.
+The files will be included immediately after the toolchain file has been read
+(if one is specified) and platform variables have been set, but before any
+languages have been enabled. Therefore, language-specific variables,
+including things like :variable:`CMAKE_<LANG>_COMPILER`, might not be set.
+See :ref:`Code Injection` for a more detailed discussion of files potentially
+included during a :command:`project` call.
+
+This variable is intended for specifying files that perform one-time setup
+for the build. It provides an injection point for things like configuring
+package managers, adding logic the user shares between projects (e.g. defining
+their own custom build types), and so on. It is primarily for users to add
+things specific to their environment, but not for specifying the toolchain
+details (use :variable:`CMAKE_TOOLCHAIN_FILE` for that).
+
+By default, this variable is empty.  It is intended to be set by the user.
+
+See also the :variable:`CMAKE_PROJECT_INCLUDE`,
+:variable:`CMAKE_PROJECT_INCLUDE_BEFORE`,
+:variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE`, and
+:variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE_BEFORE` variables.
diff --git a/Help/variable/CMAKE_TOOLCHAIN_FILE.rst b/Help/variable/CMAKE_TOOLCHAIN_FILE.rst
index ff8d59a..1117c1f 100644
--- a/Help/variable/CMAKE_TOOLCHAIN_FILE.rst
+++ b/Help/variable/CMAKE_TOOLCHAIN_FILE.rst
@@ -13,3 +13,6 @@
 
 This is initialized by the :envvar:`CMAKE_TOOLCHAIN_FILE` environment
 variable if it is set when a new build tree is first created.
+
+See the :variable:`CMAKE_PROJECT_TOP_LEVEL_INCLUDES` variable for setting
+other things not directly related to the toolchain.
diff --git a/Modules/CMakeDetermineSystem.cmake b/Modules/CMakeDetermineSystem.cmake
index 8c7af06..2c2c2ac 100644
--- a/Modules/CMakeDetermineSystem.cmake
+++ b/Modules/CMakeDetermineSystem.cmake
@@ -128,7 +128,6 @@
     set(CMAKE_TOOLCHAIN_FILE "${_INCLUDED_TOOLCHAIN_FILE}" CACHE FILEPATH "The CMake toolchain file" FORCE)
   else()
     message(FATAL_ERROR "Could not find toolchain file: ${CMAKE_TOOLCHAIN_FILE}")
-    set(CMAKE_TOOLCHAIN_FILE "NOTFOUND" CACHE FILEPATH "The CMake toolchain file" FORCE)
   endif()
 endif()
 
diff --git a/Source/cmGlobalGenerator.cxx b/Source/cmGlobalGenerator.cxx
index 09e2abe..3831546 100644
--- a/Source/cmGlobalGenerator.cxx
+++ b/Source/cmGlobalGenerator.cxx
@@ -688,6 +688,33 @@
     if (!this->FindMakeProgram(mf)) {
       return;
     }
+
+    // One-time includes of user-provided project setup files
+    std::string includes =
+      mf->GetSafeDefinition("CMAKE_PROJECT_TOP_LEVEL_INCLUDES");
+    std::vector<std::string> includesList = cmExpandedList(includes);
+    for (std::string const& setupFile : includesList) {
+      std::string absSetupFile = cmSystemTools::CollapseFullPath(
+        setupFile, mf->GetCurrentSourceDirectory());
+      if (!cmSystemTools::FileExists(absSetupFile)) {
+        cmSystemTools::Error(
+          "CMAKE_PROJECT_TOP_LEVEL_INCLUDES file does not exist: " +
+          setupFile);
+        return;
+      }
+      if (cmSystemTools::FileIsDirectory(absSetupFile)) {
+        cmSystemTools::Error(
+          "CMAKE_PROJECT_TOP_LEVEL_INCLUDES file is a directory: " +
+          setupFile);
+        return;
+      }
+      if (!mf->ReadListFile(absSetupFile)) {
+        cmSystemTools::Error(
+          "Failed reading CMAKE_PROJECT_TOP_LEVEL_INCLUDES file: " +
+          setupFile);
+        return;
+      }
+    }
   }
 
   // Check that the languages are supported by the generator and its
diff --git a/Tests/RunCMake/project/CodeInjection-stdout.txt b/Tests/RunCMake/project/CodeInjection-stdout.txt
new file mode 100644
index 0000000..88ac966
--- /dev/null
+++ b/Tests/RunCMake/project/CodeInjection-stdout.txt
@@ -0,0 +1,10 @@
+(-- )?Included CMAKE_PROJECT_INCLUDE_BEFORE
+(-- )?Included CMAKE_TOOLCHAIN_FILE
+.*Included CMAKE_PROJECT_TOP_LEVEL_INCLUDES first file
+(-- )?Included CMAKE_PROJECT_TOP_LEVEL_INCLUDES second file
+(-- )?Included CMAKE_PROJECT_INCLUDE
+(-- )?Calling sub-project
+(-- )?Included CMAKE_PROJECT_INCLUDE_BEFORE
+(-- )?Included CMAKE_PROJECT_SubProj_INCLUDE_BEFORE
+(-- )?Included CMAKE_PROJECT_INCLUDE
+(-- )?Included CMAKE_PROJECT_SubProj_INCLUDE
diff --git a/Tests/RunCMake/project/CodeInjection.cmake b/Tests/RunCMake/project/CodeInjection.cmake
new file mode 100644
index 0000000..dcf56a1
--- /dev/null
+++ b/Tests/RunCMake/project/CodeInjection.cmake
@@ -0,0 +1 @@
+add_subdirectory(CodeInjection)
diff --git a/Tests/RunCMake/project/CodeInjection/CMakeLists.txt b/Tests/RunCMake/project/CodeInjection/CMakeLists.txt
new file mode 100644
index 0000000..8ee99d0
--- /dev/null
+++ b/Tests/RunCMake/project/CodeInjection/CMakeLists.txt
@@ -0,0 +1,2 @@
+message(STATUS "Calling sub-project")
+project(SubProj LANGUAGES NONE)
diff --git a/Tests/RunCMake/project/CodeInjection/cmake_project_include.cmake b/Tests/RunCMake/project/CodeInjection/cmake_project_include.cmake
new file mode 100644
index 0000000..f3f0a7e
--- /dev/null
+++ b/Tests/RunCMake/project/CodeInjection/cmake_project_include.cmake
@@ -0,0 +1 @@
+message(STATUS "Included CMAKE_PROJECT_INCLUDE")
diff --git a/Tests/RunCMake/project/CodeInjection/cmake_project_include_before.cmake b/Tests/RunCMake/project/CodeInjection/cmake_project_include_before.cmake
new file mode 100644
index 0000000..01d53c9
--- /dev/null
+++ b/Tests/RunCMake/project/CodeInjection/cmake_project_include_before.cmake
@@ -0,0 +1 @@
+message(STATUS "Included CMAKE_PROJECT_INCLUDE_BEFORE")
diff --git a/Tests/RunCMake/project/CodeInjection/cmake_project_subproj_include.cmake b/Tests/RunCMake/project/CodeInjection/cmake_project_subproj_include.cmake
new file mode 100644
index 0000000..d68de6a
--- /dev/null
+++ b/Tests/RunCMake/project/CodeInjection/cmake_project_subproj_include.cmake
@@ -0,0 +1 @@
+message(STATUS "Included CMAKE_PROJECT_SubProj_INCLUDE")
diff --git a/Tests/RunCMake/project/CodeInjection/cmake_project_subproj_include_before.cmake b/Tests/RunCMake/project/CodeInjection/cmake_project_subproj_include_before.cmake
new file mode 100644
index 0000000..ef3bfc0
--- /dev/null
+++ b/Tests/RunCMake/project/CodeInjection/cmake_project_subproj_include_before.cmake
@@ -0,0 +1 @@
+message(STATUS "Included CMAKE_PROJECT_SubProj_INCLUDE_BEFORE")
diff --git a/Tests/RunCMake/project/CodeInjection/cmake_project_top_level_includes_1.cmake b/Tests/RunCMake/project/CodeInjection/cmake_project_top_level_includes_1.cmake
new file mode 100644
index 0000000..73ad037
--- /dev/null
+++ b/Tests/RunCMake/project/CodeInjection/cmake_project_top_level_includes_1.cmake
@@ -0,0 +1 @@
+message(STATUS "Included CMAKE_PROJECT_TOP_LEVEL_INCLUDES first file")
diff --git a/Tests/RunCMake/project/CodeInjection/cmake_project_top_level_includes_2.cmake b/Tests/RunCMake/project/CodeInjection/cmake_project_top_level_includes_2.cmake
new file mode 100644
index 0000000..80f9705
--- /dev/null
+++ b/Tests/RunCMake/project/CodeInjection/cmake_project_top_level_includes_2.cmake
@@ -0,0 +1 @@
+message(STATUS "Included CMAKE_PROJECT_TOP_LEVEL_INCLUDES second file")
diff --git a/Tests/RunCMake/project/CodeInjection/initial_cache.cmake b/Tests/RunCMake/project/CodeInjection/initial_cache.cmake
new file mode 100644
index 0000000..6c8995b
--- /dev/null
+++ b/Tests/RunCMake/project/CodeInjection/initial_cache.cmake
@@ -0,0 +1,10 @@
+set(CMAKE_TOOLCHAIN_FILE                 "${CMAKE_CURRENT_LIST_DIR}/passthrough_toolchain_file.cmake" CACHE FILEPATH "")
+set(CMAKE_PROJECT_INCLUDE                "${CMAKE_CURRENT_LIST_DIR}/cmake_project_include.cmake" CACHE FILEPATH "")
+set(CMAKE_PROJECT_INCLUDE_BEFORE         "${CMAKE_CURRENT_LIST_DIR}/cmake_project_include_before.cmake" CACHE FILEPATH "")
+set(CMAKE_PROJECT_SubProj_INCLUDE        "${CMAKE_CURRENT_LIST_DIR}/cmake_project_subproj_include.cmake" CACHE FILEPATH "")
+set(CMAKE_PROJECT_SubProj_INCLUDE_BEFORE "${CMAKE_CURRENT_LIST_DIR}/cmake_project_subproj_include_before.cmake" CACHE FILEPATH "")
+set(CMAKE_PROJECT_TOP_LEVEL_INCLUDES
+  "${CMAKE_CURRENT_LIST_DIR}/cmake_project_top_level_includes_1.cmake"
+  "${CMAKE_CURRENT_LIST_DIR}/cmake_project_top_level_includes_2.cmake"
+  CACHE STRING ""
+)
diff --git a/Tests/RunCMake/project/CodeInjection/passthrough_toolchain_file.cmake b/Tests/RunCMake/project/CodeInjection/passthrough_toolchain_file.cmake
new file mode 100644
index 0000000..d045712
--- /dev/null
+++ b/Tests/RunCMake/project/CodeInjection/passthrough_toolchain_file.cmake
@@ -0,0 +1 @@
+message(STATUS "Included CMAKE_TOOLCHAIN_FILE")
diff --git a/Tests/RunCMake/project/RunCMakeTest.cmake b/Tests/RunCMake/project/RunCMakeTest.cmake
index d7dab12..945d9ed 100644
--- a/Tests/RunCMake/project/RunCMakeTest.cmake
+++ b/Tests/RunCMake/project/RunCMakeTest.cmake
@@ -1,5 +1,14 @@
 include(RunCMake)
 
+# Use an initial cache file to define the project() variables
+# to avoid long command lines. Also see the CMakeOnly test case
+# which tests some of the individual variables one at a time.
+# Here, we are focused on testing that the variables are all injected
+# at the expected points in the expected order.
+run_cmake_with_options(CodeInjection
+  -C "${CMAKE_CURRENT_LIST_DIR}/CodeInjection/initial_cache.cmake"
+)
+
 if(CMake_TEST_RESOURCES)
   run_cmake(ExplicitRC)
 endif()