diff -r 525320df8afb -r ebe5760afa08 Doc/library/site.rst
--- a/Doc/library/site.rst	Thu Mar 17 20:20:30 2011 +0200
+++ b/Doc/library/site.rst	Mon Mar 21 13:40:17 2011 -0400
@@ -13,7 +13,11 @@
 
 .. index:: triple: module; search; path
 
-Importing this module will append site-specific paths to the module search path.
+Importing this module will append site-specific paths to the module search
+path, unless :option:`-S` was used. In that case, this module can be safely
+imported with no automatic modifications to the module search path. To
+explicitly trigger the usual site-specific additions, call this module's
+:func:`main` function.
 
 .. index::
    pair: site-python; directory
@@ -114,6 +118,12 @@
 .. envvar:: PYTHONUSERBASE
 
 
+.. function:: main()
+
+   Adds all the standard site-specific directories to the module search
+   path. This function is called automatically when this module is imported,
+   unless the python interpreter was run with the :option:`-S` flag.
+
 .. function:: addsitedir(sitedir, known_paths=None)
 
    Adds a directory to sys.path and processes its pth files.
diff -r 525320df8afb -r ebe5760afa08 Lib/site.py
--- a/Lib/site.py	Thu Mar 17 20:20:30 2011 +0200
+++ b/Lib/site.py	Mon Mar 21 13:40:17 2011 -0400
@@ -526,7 +526,8 @@
     if ENABLE_USER_SITE:
         execusercustomize()
 
-main()
+if not sys.flags.no_site:
+    main()
 
 def _script():
     help = """\
diff -r 525320df8afb -r ebe5760afa08 Misc/NEWS
--- a/Misc/NEWS	Thu Mar 17 20:20:30 2011 +0200
+++ b/Misc/NEWS	Mon Mar 21 13:40:17 2011 -0400
@@ -72,6 +72,9 @@
 Library
 -------
 
+- Issue #11591: Prevent imports from "site" from automatically adding site
+  directories to sys.path under "python -S".
+
 - Issue #10812: Add some extra posix functions to the os module.
 
 - Issue #10979: unittest stdout buffering now works with class and module