diff --git a/Lib/inspect.py b/Lib/inspect.py
index 227f6b8a63bd9b..dd3b784a607e9d 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -1973,7 +1973,7 @@ def _signature_fromstr(cls, obj, s, skip_bound_arg=True):
         module = sys.modules.get(module_name, None)
         if module:
             module_dict = module.__dict__
-    sys_module_dict = sys.modules
+    sys_module_dict = sys.modules.copy()
 
     def parse_name(node):
         assert isinstance(node, ast.arg)
diff --git a/Misc/NEWS.d/next/Library/2018-10-04-18-46-54.bpo-34871.t3X-dB.rst b/Misc/NEWS.d/next/Library/2018-10-04-18-46-54.bpo-34871.t3X-dB.rst
new file mode 100644
index 00000000000000..8cff15671ce536
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-10-04-18-46-54.bpo-34871.t3X-dB.rst
@@ -0,0 +1,2 @@
+Fix inspect module polluted ``sys.modules`` when parsing
+``__text_signature__`` of callable.