diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -329,7 +329,8 @@
 
 .. c:function:: void PySys_SetArgv(int argc, wchar_t **argv)
 
-   This function works like :c:func:`PySys_SetArgvEx` with *updatepath* set to 1.
+   This function works like :c:func:`PySys_SetArgvEx` with *updatepath* set
+   to 1 unless the :program:`python` interpreter was started with the :option:`-I`.
 
 
 .. c:function:: void Py_SetPythonHome(wchar_t *home)
diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -24,7 +24,7 @@
 
 When invoking Python, you may specify any of these options::
 
-    python [-bBdEhiOqsSuvVWx?] [-c command | -m module-name | script | - ] [args]
+    python [-bBdEhiIOqsSuvVWx?] [-c command | -m module-name | script | - ] [args]
 
 The most common use case is, of course, a simple invocation of a script::
 
@@ -208,6 +208,15 @@
    raises an exception.  See also :envvar:`PYTHONINSPECT`.
 
 
+.. cmdoption:: -I
+
+   Run Python in isolated mode. This also implies -E and -s.
+   In isolated mode :data:`sys.path` contains neither the script's directory nor
+   the user's site-packages directory. All :envvar:`PYTHON*` environment
+   variables are ignored, too. Further restrictions may be imposed to prevent
+   the user from injecting malicious code.
+
+
 .. cmdoption:: -O
 
    Turn on basic optimizations.  This changes the filename extension for
@@ -382,7 +391,7 @@
 ---------------------
 
 These environment variables influence Python's behavior, they are processed
-before the command-line switches other than -E.  It is customary that
+before the command-line switches other than -E or -I.  It is customary that
 command-line switches override environmental variables where there is a
 conflict.
 
diff --git a/Include/pydebug.h b/Include/pydebug.h
--- a/Include/pydebug.h
+++ b/Include/pydebug.h
@@ -20,6 +20,7 @@
 PyAPI_DATA(int) Py_NoUserSiteDirectory;
 PyAPI_DATA(int) Py_UnbufferedStdioFlag;
 PyAPI_DATA(int) Py_HashRandomizationFlag;
+PyAPI_DATA(int) Py_IsolatedFlag;
 
 /* this is a wrapper around getenv() that pays attention to
    Py_IgnoreEnvironmentFlag.  It should be used for getting variables like
diff --git a/Modules/main.c b/Modules/main.c
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -43,7 +43,7 @@
 static int  orig_argc;
 
 /* command line options */
-#define BASE_OPTS L"bBc:dEhiJm:OqRsStuvVW:xX:?"
+#define BASE_OPTS L"bBc:dEhiIJm:OqRsStuvVW:xX:?"
 
 #define PROGRAM_OPTS BASE_OPTS
 
@@ -65,6 +65,7 @@
 static char *usage_2 = "\
 -i     : inspect interactively after running script; forces a prompt even\n\
          if stdin does not appear to be a terminal; also PYTHONINSPECT=x\n\
+-I     : isolate Python from the user's enviroment (implies -E and -s)\n\
 -m mod : run library module as a script (terminates option list)\n\
 -O     : optimize generated bytecode slightly; also PYTHONOPTIMIZE=x\n\
 -OO    : remove doc-strings in addition to the -O optimizations\n\
@@ -392,6 +393,12 @@
             Py_InteractiveFlag++;
             break;
 
+        case 'I':
+            Py_IsolatedFlag++;
+            Py_NoUserSiteDirectory++;
+            Py_IgnoreEnvironmentFlag++;
+            break;
+
         /* case 'J': reserved for Jython */
 
         case 'O':
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -93,6 +93,7 @@
 int Py_NoUserSiteDirectory = 0; /* for -s and site.py */
 int Py_UnbufferedStdioFlag = 0; /* Unbuffered binary std{in,out,err} */
 int Py_HashRandomizationFlag = 0; /* for -R and PYTHONHASHSEED */
+int Py_IsolatedFlag = 0; /* for -I, isolate from user's env */
 
 PyThreadState *_Py_Finalizing = NULL;
 
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1357,6 +1357,7 @@
     {"bytes_warning",           "-b"},
     {"quiet",                   "-q"},
     {"hash_randomization",      "-R"},
+    {"isolated",                "-I"},
     {0}
 };
 
@@ -1365,9 +1366,9 @@
     flags__doc__,       /* doc */
     flags_fields,       /* fields */
 #ifdef RISCOS
+    14
+#else
     13
-#else
-    12
 #endif
 };
 
@@ -1401,6 +1402,7 @@
     SetFlag(Py_BytesWarningFlag);
     SetFlag(Py_QuietFlag);
     SetFlag(Py_HashRandomizationFlag);
+    SetFlag(Py_IsolatedFlag);
 #undef SetFlag
 
     if (PyErr_Occurred()) {
@@ -1925,7 +1927,7 @@
 void
 PySys_SetArgv(int argc, wchar_t **argv)
 {
-    PySys_SetArgvEx(argc, argv, 1);
+    PySys_SetArgvEx(argc, argv, Py_IsolatedFlag == 0);
 }
 
 /* Reimplementation of PyFile_WriteString() no calling indirectly