diff --git a/Misc/NEWS.d/next/Build/2026-11-06-12-00-00.gh-issue-140266.K3hP9z.rst b/Misc/NEWS.d/next/Build/2026-11-06-12-00-00.gh-issue-140266.K3hP9z.rst
new file mode 100644
index 000000000000000..399aca8a9bcff6c
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2026-11-06-12-00-00.gh-issue-140266.K3hP9z.rst
@@ -0,0 +1,3 @@
+Fix ``"platform": ""`` in ``build-details.json`` for native builds configured
+with :option:`--with-build-python`: :envvar:`!_PYTHON_HOST_PLATFORM` is now
+only passed to the build interpreter when cross compiling.
diff --git a/configure b/configure
index fca5669c3756b4d..9abbe58a86d375a 100755
--- a/configure
+++ b/configure
@@ -3829,7 +3829,12 @@ fi
     fi
         ac_cv_prog_PYTHON_FOR_REGEN=$with_build_python
     PYTHON_FOR_FREEZE="$with_build_python"
-    PYTHON_FOR_BUILD='_PYTHON_PROJECT_BASE=$(abs_builddir) _PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) PYTHONPATH=$(srcdir)/Lib _PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata_$(ABIFLAGS)_$(MACHDEP)_$(MULTIARCH) _PYTHON_SYSCONFIGDATA_PATH=$(shell test -f pybuilddir.txt && echo $(abs_builddir)/`cat pybuilddir.txt`) '$with_build_python
+    PYTHON_FOR_BUILD='_PYTHON_PROJECT_BASE=$(abs_builddir) PYTHONPATH=$(srcdir)/Lib _PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata_$(ABIFLAGS)_$(MACHDEP)_$(MULTIARCH) _PYTHON_SYSCONFIGDATA_PATH=$(shell test -f pybuilddir.txt && echo $(abs_builddir)/`cat pybuilddir.txt`) '$with_build_python
+        if test "x$cross_compiling" = xyes
+then :
+  PYTHON_FOR_BUILD='_PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) '"$PYTHON_FOR_BUILD"
+
+fi
     { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $with_build_python" >&5
 printf "%s\n" "$with_build_python" >&6; }
 
diff --git a/configure.ac b/configure.ac
index 753f997a20da4a2..6d7ed9b77da339e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -170,7 +170,11 @@ AC_ARG_WITH([build-python],
     dnl Build Python interpreter is used for regeneration and freezing.
     ac_cv_prog_PYTHON_FOR_REGEN=$with_build_python
     PYTHON_FOR_FREEZE="$with_build_python"
-    PYTHON_FOR_BUILD='_PYTHON_PROJECT_BASE=$(abs_builddir) _PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) PYTHONPATH=$(srcdir)/Lib _PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata_$(ABIFLAGS)_$(MACHDEP)_$(MULTIARCH) _PYTHON_SYSCONFIGDATA_PATH=$(shell test -f pybuilddir.txt && echo $(abs_builddir)/`cat pybuilddir.txt`) '$with_build_python
+    PYTHON_FOR_BUILD='_PYTHON_PROJECT_BASE=$(abs_builddir) PYTHONPATH=$(srcdir)/Lib _PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata_$(ABIFLAGS)_$(MACHDEP)_$(MULTIARCH) _PYTHON_SYSCONFIGDATA_PATH=$(shell test -f pybuilddir.txt && echo $(abs_builddir)/`cat pybuilddir.txt`) '$with_build_python
+    dnl _PYTHON_HOST_PLATFORM is only set when cross compiling.
+    AS_VAR_IF([cross_compiling], [yes],
+      [PYTHON_FOR_BUILD='_PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) '"$PYTHON_FOR_BUILD"]
+    )
     AC_MSG_RESULT([$with_build_python])
   ], [
     AS_VAR_IF([cross_compiling], [yes],