diff --git a/CHANGELOG.md b/CHANGELOG.md
index 02174e280..015457325 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - jrl_release: add `ConanFileVersionExtractor` by @arntanguy
 - jrl_release: add `--check-version --check-tag vX.Y.Z` by @arntanguy
 - jrl_release: add `DebianChangelogVersionExtractor` by @arntanguy
+- jrl: change jrl_configure_default_install_dirs() to macro to make GNUInstallDirs in caller's scope by @ahoarau
 
 ## [2.1.0] - 2026-07-03
 
diff --git a/v2/docs/api.md b/v2/docs/api.md
index 90693e850..861a38a68 100644
--- a/v2/docs/api.md
+++ b/v2/docs/api.md
@@ -219,13 +219,17 @@ jrl_target_set_output_directory(my_python_module_target OUTPUT_DIRECTORY ${CMAKE
 jrl_configure_default_install_dirs()
 ```
 
-**Type:** function
+**Type:** macro
 
 
 ### Description
   Configures the default install directories using GNUInstallDirs (bin, lib, include, etc.).
   Works on all platforms.
 
+  Must be called directly from a project's `CMakeLists.txt`, not wrapped in a `function()`:
+  `CMAKE_INSTALL_DOCDIR`, `DATADIR`, `MANDIR`, `INFODIR`, `LOCALEDIR` and every
+  `CMAKE_INSTALL_FULL_*` are not cache entries and would be lost when the function returns.
+
 
 ### Arguments
   None
diff --git a/v2/modules/jrl.cmake b/v2/modules/jrl.cmake
index 6caea223b..cb4c62ea4 100644
--- a/v2/modules/jrl.cmake
+++ b/v2/modules/jrl.cmake
@@ -796,13 +796,17 @@ endfunction()
 jrl_configure_default_install_dirs()
 ```
 
-**Type:** function
+**Type:** macro
 
 
 ### Description
   Configures the default install directories using GNUInstallDirs (bin, lib, include, etc.).
   Works on all platforms.
 
+  Must be called directly from a project's `CMakeLists.txt`, not wrapped in a `function()`:
+  `CMAKE_INSTALL_DOCDIR`, `DATADIR`, `MANDIR`, `INFODIR`, `LOCALEDIR` and every
+  `CMAKE_INSTALL_FULL_*` are not cache entries and would be lost when the function returns.
+
 
 ### Arguments
   None
@@ -813,19 +817,37 @@ jrl_configure_default_install_dirs()
 jrl_configure_default_install_dirs()
 ```
 #]============================================================================]
-function(jrl_configure_default_install_dirs)
+macro(jrl_configure_default_install_dirs)
+    if(DEFINED CMAKE_CURRENT_FUNCTION)
+        message(
+            FATAL_ERROR
+            "jrl_configure_default_install_dirs() must be called directly from a CMakeLists.txt, not from inside function '${CMAKE_CURRENT_FUNCTION}()'.
+            The GNUInstallDirs variables that are not cache entries (CMAKE_INSTALL_DOCDIR, DATADIR, MANDIR,
+            INFODIR, LOCALEDIR and all CMAKE_INSTALL_FULL_*) would be lost when that function returns.
+            "
+        )
+    endif()
+
     # Prevent the following warning on pure-cmake projects (i.e. defined with LANGUAGES NONE, like jrl-cmakemodules):
     # "Unable to determine default CMAKE_INSTALL_LIBDIR directory because no target architecture is known.
     # Please enable at least one language before including GNUInstallDirs"
     # ref: https://github.com/Kitware/CMake/blob/v4.2.3/Modules/GNUInstallDirs.cmake#L434C8-L435C75
     # issue: https://gitlab.kitware.com/cmake/cmake/-/issues/23461
-
-    if(NOT DEFINED CMAKE_SIZEOF_VOID_P)
+    if(DEFINED CMAKE_SIZEOF_VOID_P)
+        set(_cmake_sizeof_already_defined false)
+    else()
+        set(_cmake_sizeof_already_defined true)
         set(CMAKE_SIZEOF_VOID_P 0)
     endif()
 
     include(GNUInstallDirs)
-endfunction()
+
+    # Do not leak the placeholder: a defined-but-0 value is worse than an undefined one.
+    if(_cmake_sizeof_already_defined)
+        unset(CMAKE_SIZEOF_VOID_P)
+    endif()
+    unset(_cmake_sizeof_already_defined)
+endmacro()
 
 #[============================================================================[
 # `jrl_configure_default_install_prefix`
@@ -939,8 +961,9 @@ jrl_configure_defaults()
 macro(jrl_configure_defaults)
     jrl_configure_default_build_type(Release)
     jrl_configure_default_binary_dirs()
-    jrl_configure_default_install_dirs()
+    # Before install_dirs: GNUInstallDirs derives CMAKE_INSTALL_FULL_* from the prefix, once.
     jrl_configure_default_install_prefix(${CMAKE_BINARY_DIR}/install)
+    jrl_configure_default_install_dirs()
     jrl_configure_copy_compile_commands_in_source_dir()
     jrl_configure_uninstall_target()
 endmacro()
diff --git a/v2/tests/CMakeLists.txt b/v2/tests/CMakeLists.txt
index 89ede6354..b5add8aec 100644
--- a/v2/tests/CMakeLists.txt
+++ b/v2/tests/CMakeLists.txt
@@ -39,3 +39,4 @@ add_subdirectory(print_dependencies_summary)
 add_subdirectory(export_package_control)
 add_subdirectory(target_headers_relative)
 add_subdirectory(output_dirs)
+add_subdirectory(gnu_install_dirs)
diff --git a/v2/tests/gnu_install_dirs/CMakeLists.txt b/v2/tests/gnu_install_dirs/CMakeLists.txt
new file mode 100644
index 000000000..14bb1a66f
--- /dev/null
+++ b/v2/tests/gnu_install_dirs/CMakeLists.txt
@@ -0,0 +1 @@
+add_cmake_test(NAME gid-project DEPENDS jrl-cmakemodules)
diff --git a/v2/tests/gnu_install_dirs/README.md b/v2/tests/gnu_install_dirs/README.md
new file mode 100644
index 000000000..4ea558eb9
--- /dev/null
+++ b/v2/tests/gnu_install_dirs/README.md
@@ -0,0 +1,6 @@
+# gnu_install_dirs
+
+Verifies that `jrl_configure_defaults()` leaves every `GNUInstallDirs` variable usable in the
+caller's scope, including `CMAKE_INSTALL_DOCDIR`, `DATADIR`, `MANDIR`, `INFODIR`, `LOCALEDIR`
+and all the `CMAKE_INSTALL_FULL_*`, which are not cache entries and are therefore lost if
+`GNUInstallDirs` is included from inside a `function()`.
diff --git a/v2/tests/gnu_install_dirs/gid-project/CMakeLists.txt b/v2/tests/gnu_install_dirs/gid-project/CMakeLists.txt
new file mode 100644
index 000000000..3f0654346
--- /dev/null
+++ b/v2/tests/gnu_install_dirs/gid-project/CMakeLists.txt
@@ -0,0 +1,61 @@
+cmake_minimum_required(VERSION 3.22)
+
+project(gid-project VERSION 1.0.0 LANGUAGES CXX)
+
+find_package(jrl-cmakemodules 2.0.0 CONFIG REQUIRED)
+
+jrl_configure_defaults()
+
+# GNUInstallDirs only creates cache entries for its root dirs (BINDIR, LIBDIR, ...).
+# The derived ones (DATADIR, DOCDIR, MANDIR, INFODIR, LOCALEDIR) and every
+# CMAKE_INSTALL_FULL_* are plain variables, lost if GNUInstallDirs is included
+# from inside a function(). They must all be usable in the caller's scope.
+set(expected_install_dirs
+    BINDIR
+    SBINDIR
+    LIBEXECDIR
+    SYSCONFDIR
+    SHAREDSTATEDIR
+    LOCALSTATEDIR
+    LIBDIR
+    INCLUDEDIR
+    OLDINCLUDEDIR
+    DATAROOTDIR
+    DATADIR
+    INFODIR
+    LOCALEDIR
+    MANDIR
+    DOCDIR
+)
+
+foreach(dir IN LISTS expected_install_dirs)
+    foreach(var CMAKE_INSTALL_${dir} CMAKE_INSTALL_FULL_${dir})
+        if(NOT DEFINED ${var})
+            message(FATAL_ERROR "${var} is not defined after jrl_configure_defaults()")
+        endif()
+        if(${var} STREQUAL "")
+            message(FATAL_ERROR "${var} is empty after jrl_configure_defaults()")
+        endif()
+    endforeach()
+    message(
+        STATUS
+        "CMAKE_INSTALL_${dir}=${CMAKE_INSTALL_${dir}} (full: ${CMAKE_INSTALL_FULL_${dir}})"
+    )
+endforeach()
+
+if(NOT CMAKE_INSTALL_DOCDIR STREQUAL "${CMAKE_INSTALL_DATAROOTDIR}/doc/${PROJECT_NAME}")
+    message(
+        FATAL_ERROR
+        "Expected CMAKE_INSTALL_DOCDIR to be '${CMAKE_INSTALL_DATAROOTDIR}/doc/${PROJECT_NAME}', got: '${CMAKE_INSTALL_DOCDIR}'"
+    )
+endif()
+
+file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/test.txt" "OK\n")
+install(FILES "${CMAKE_CURRENT_BINARY_DIR}/test.txt" DESTINATION ${CMAKE_INSTALL_DOCDIR})
+
+enable_testing()
+
+add_test(
+    NAME "gnu_install_dirs: test.txt is installed in CMAKE_INSTALL_DOCDIR"
+    COMMAND ${CMAKE_COMMAND} -E cat "${CMAKE_INSTALL_FULL_DOCDIR}/test.txt"
+)
diff --git a/v2/tests/gnu_install_dirs/gid-project/pixi.toml b/v2/tests/gnu_install_dirs/gid-project/pixi.toml
new file mode 100644
index 000000000..d5fabe591
--- /dev/null
+++ b/v2/tests/gnu_install_dirs/gid-project/pixi.toml
@@ -0,0 +1,30 @@
+[workspace]
+name = "gid-project"
+version = "1.0.0"
+channels = ["conda-forge"]
+platforms = ["osx-arm64", "osx-64", "linux-64", "linux-aarch64", "win-64"]
+preview = ["pixi-build"]
+
+[dependencies]
+cxx-compiler = "*"
+cmake = ">=3.22"
+ninja = "*"
+jrl-cmakemodules = { path = "../../../../" }
+
+[tasks]
+clear = { cmd = "rm -rf build" }
+configure = { cmd = "cmake --log-level=DEBUG -G Ninja -S . -B build" }
+build = { cmd = "cmake --build build" }
+install = { cmd = "cmake --install build" }
+ctest = { cmd = "ctest --test-dir build --output-on-failure --no-tests=ignore" }
+test = { depends-on = ["clear", "configure", "build", "install", "ctest"] }
+
+[package]
+name = { workspace = true }
+version = { workspace = true }
+
+[package.host-dependencies]
+jrl-cmakemodules = { path = "../../../.." }
+
+[package.build]
+backend = { name = "pixi-build-cmake", version = "*" }
diff --git a/v2/tests/gnu_install_dirs/pixi.toml b/v2/tests/gnu_install_dirs/pixi.toml
new file mode 100644
index 000000000..e0904c319
--- /dev/null
+++ b/v2/tests/gnu_install_dirs/pixi.toml
@@ -0,0 +1,6 @@
+[workspace]
+channels = ["conda-forge"]
+platforms = ["linux-64", "linux-aarch64", "osx-arm64", "osx-64", "win-64"]
+
+[tasks]
+test = { cmd = "pixi run test", cwd = "gid-project" }
diff --git a/v2/tests/pixi.toml b/v2/tests/pixi.toml
index 089c688eb..216457101 100644
--- a/v2/tests/pixi.toml
+++ b/v2/tests/pixi.toml
@@ -25,6 +25,7 @@ test_header_visibility = { cmd = "pixi run test", cwd = "header_visibility" }
 test_export_package_control = { cmd = "pixi run test", cwd = "export_package_control" }
 test_target_headers_relative = { cmd = "pixi run test", cwd = "target_headers_relative" }
 test_output_dirs = { cmd = "pixi run test", cwd = "output_dirs" }
+test_gnu_install_dirs = { cmd = "pixi run test", cwd = "gnu_install_dirs" }
 
 test = { depends-on = [
     "test_find_modules",
@@ -47,4 +48,5 @@ test = { depends-on = [
     "test_export_package_control",
     "test_target_headers_relative",
     "test_output_dirs",
+    "test_gnu_install_dirs",
 ] }