diff --git a/var/spack/repos/builtin/packages/tcl/package.py b/var/spack/repos/builtin/packages/tcl/package.py
index 5d5419a5c99..08c592a68a5 100644
--- a/var/spack/repos/builtin/packages/tcl/package.py
+++ b/var/spack/repos/builtin/packages/tcl/package.py
@@ -182,6 +182,12 @@ def setup_dependent_build_environment(self, env, dependent_spec):
 class AutotoolsBuilder(AnyBuilder, spack.build_systems.autotools.AutotoolsBuilder):
     configure_directory = "unix"
 
+    # if TCL is present on the system this may be set to the system's
+    # existing TCL so ensure it is unset
+    # https://wiki.tcl-lang.org/page/TCL%5FLIBRARY
+    def setup_build_environment(self, env):
+        env.set("TCL_LIBRARY", "")
+
     def install(self, pkg, spec, prefix):
         with working_dir(self.build_directory):
             make("install")