diff --git a/cmake/modules/RootConfiguration.cmake b/cmake/modules/RootConfiguration.cmake
index 1fd7e8a496185..489281df231fd 100644
--- a/cmake/modules/RootConfiguration.cmake
+++ b/cmake/modules/RootConfiguration.cmake
@@ -548,6 +548,19 @@ endif()
 string(REGEX MATCH "__cplusplus[=| ]([0-9]+)" __cplusplus "${__cplusplus_PPout}")
 set(__cplusplus ${CMAKE_MATCH_1}L)
 
+# To mark the build tree. Important for automatic resolution of relative paths,
+# for example to the include directory. Use custom target to ensure re-creation
+# when someone deletes the marker.
+set(build_tree_marker "${localruntimedir}/root-build-tree-marker")
+add_custom_command(
+  OUTPUT "${build_tree_marker}"
+  COMMAND ${CMAKE_COMMAND} -E touch "${build_tree_marker}"
+  COMMENT "Ensuring that \"${build_tree_marker}\" exists"
+)
+add_custom_target(ensure_build_tree_marker ALL
+  DEPENDS "${build_tree_marker}"
+)
+
 configure_file(${PROJECT_SOURCE_DIR}/config/RConfigure.in ginclude/RConfigure.h NEWLINE_STYLE UNIX)
 install(FILES ${CMAKE_BINARY_DIR}/ginclude/RConfigure.h DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 
diff --git a/config/RConfigure.in b/config/RConfigure.in
index d8fa08521d0c9..09931572afe88 100644
--- a/config/RConfigure.in
+++ b/config/RConfigure.in
@@ -9,7 +9,6 @@
 #define ROOTPREFIX    "@prefix@"
 #define ROOTBINDIR    "@bindir@"
 #define ROOTLIBDIR    "@libdir@"
-#define ROOTINCDIR    "@incdir@"
 #define ROOTETCDIR    "@etcdir@"
 #define ROOTDATADIR   "@datadir@"
 #define ROOTDOCDIR    "@docdir@"
diff --git a/core/base/CMakeLists.txt b/core/base/CMakeLists.txt
index 97fffc3e9b6bb..26177a4d446cc 100644
--- a/core/base/CMakeLists.txt
+++ b/core/base/CMakeLists.txt
@@ -236,9 +236,21 @@ if(core_soversion)
 else()
     set(full_core_filename "${core_prefix}Core${core_suffix}")
 endif()
+
+# Absolue CMAKE_INSTALL_<dir> paths are discouraged in CMake, but some
+# packagers use them anyway. So we support it.
+if(IS_ABSOLUTE ${CMAKE_INSTALL_INCLUDEDIR})
+   set(install_includedir_is_absolute 1)
+else()
+   set(install_includedir_is_absolute 0)
+endif()
+file(TO_NATIVE_PATH "${CMAKE_INSTALL_INCLUDEDIR}" install_includedir_native)
+
 target_compile_options(Core PRIVATE -DLIB_CORE_NAME=${full_core_filename}
-                                    -DCMAKE_INSTALL_INCLUDEDIR=${CMAKE_INSTALL_INCLUDEDIR}
+                                    -DINSTALL_INCLUDEDIR=${install_includedir_native}
+                                    -DINSTALL_INCLUDEDIR_IS_ABSOLUTE=${install_includedir_is_absolute}
 )
+add_dependencies(Core ensure_build_tree_marker)
 
 if(PCRE2_FOUND)
   target_link_libraries(Core PRIVATE PCRE2::PCRE2)
diff --git a/core/base/src/TROOT.cxx b/core/base/src/TROOT.cxx
index af0068d8c1bff..f557c4d440977 100644
--- a/core/base/src/TROOT.cxx
+++ b/core/base/src/TROOT.cxx
@@ -1993,9 +1993,7 @@ void TROOT::InitSystem()
       if (gSystem->Init())
          fprintf(stderr, "Fatal in <TROOT::InitSystem>: can't init operating system layer\n");
 
-      TString rootincludedir = _R_QUOTEVAL_(CMAKE_INSTALL_INCLUDEDIR);
-      gSystem->PrependPathName(GetRootSys(), rootincludedir);
-      gSystem->SetIncludePath(("-I" + rootincludedir).Data());
+      gSystem->SetIncludePath(("-I" + GetIncludeDir()).Data());
 
       if (!gSystem->HomeDirectory()) {
          fprintf(stderr, "Fatal in <TROOT::InitSystem>: HOME directory not set\n");
@@ -3098,11 +3096,29 @@ const TString& TROOT::GetSharedLibDir() {
 ////////////////////////////////////////////////////////////////////////////////
 /// Get the include directory in the installation. Static utility function.
 
-const TString& TROOT::GetIncludeDir() {
-   // Avoid returning a reference to a temporary because of the conversion
-   // between std::string and TString.
-   const static TString includedir = ROOT::FoundationUtils::GetIncludeDir();
-   return includedir;
+const TString &TROOT::GetIncludeDir()
+{
+   static TString rootincdir;
+
+   if (!rootincdir.IsNull())
+      return rootincdir;
+
+   const std::string &sep = ROOT::FoundationUtils::GetPathSeparator();
+
+   // Check if we are in the build tree using the build tree marker file
+   const bool isBuildTree = std::filesystem::exists((GetSharedLibDir() + sep + "root-build-tree-marker").Data());
+
+   if (isBuildTree) {
+      rootincdir = GetRootSys() + sep + "include";
+   } else {
+#if INSTALL_INCLUDEDIR_IS_ABSOLUTE
+      rootincdir = _R_QUOTEVAL_(INSTALL_INCLUDEDIR);
+#else
+      rootincdir = GetRootSys() + sep + _R_QUOTEVAL_(INSTALL_INCLUDEDIR);
+#endif
+   }
+
+   return rootincdir;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
diff --git a/core/foundation/res/ROOT/FoundationUtils.hxx b/core/foundation/res/ROOT/FoundationUtils.hxx
index ce224f1067130..c4ba65cfe093c 100644
--- a/core/foundation/res/ROOT/FoundationUtils.hxx
+++ b/core/foundation/res/ROOT/FoundationUtils.hxx
@@ -67,10 +67,6 @@ namespace FoundationUtils {
    ///
    const std::string& GetRootSys();
 
-   ///\ returns the include directory in the installation.
-   ///
-   const std::string& GetIncludeDir();
-
    ///\returns the sysconfig directory in the installation.
    const std::string& GetEtcDir();
 
diff --git a/core/foundation/src/FoundationUtils.cxx b/core/foundation/src/FoundationUtils.cxx
index 702c00377c8c2..b1b73b2d47fe6 100644
--- a/core/foundation/src/FoundationUtils.cxx
+++ b/core/foundation/src/FoundationUtils.cxx
@@ -167,8 +167,10 @@ const std::string& GetRootSys() {
    if (rootsys.empty()) {
       if (const char* envValue = std::getenv("ROOTSYS")) {
          rootsys = envValue;
+#ifndef WIN32
          // We cannot use gSystem->UnixPathName.
          ConvertToUnixPath(rootsys);
+#endif
       }
    }
    // FIXME: Should this also call UnixPathName for consistency?
@@ -177,22 +179,6 @@ const std::string& GetRootSys() {
    return rootsys;
 }
 
-
-const std::string& GetIncludeDir() {
-#ifdef ROOTINCDIR
-   if (!IgnorePrefix()) {
-      const static std::string rootincdir = ROOTINCDIR;
-      return rootincdir;
-   }
-#endif
-   static std::string rootincdir;
-   if (rootincdir.empty()) {
-      const std::string& sep = GetPathSeparator();
-      rootincdir = GetRootSys() + sep + "include" + sep;
-   }
-   return rootincdir;
-}
-
 const std::string& GetEtcDir() {
 #ifdef ROOTETCDIR
    if (!IgnorePrefix()) {