diff --git a/IPython/core/debugger.py b/IPython/core/debugger.py
index 01b21bf..b7f2165 100644
--- a/IPython/core/debugger.py
+++ b/IPython/core/debugger.py
@@ -38,7 +38,7 @@ from IPython.core.excolors import exception_colors
 has_pydb = False
 prompt = 'ipdb> '
 #We have to check this directly from sys.argv, config struct not yet available
-if '-pydb' in sys.argv:
+if '--pydb' in sys.argv:
     try:
         import pydb
         if hasattr(pydb.pydb, "runl") and pydb.version>'1.17':
diff --git a/IPython/core/shellapp.py b/IPython/core/shellapp.py
index 99ebbd2..414e3a3 100644
--- a/IPython/core/shellapp.py
+++ b/IPython/core/shellapp.py
@@ -50,6 +50,14 @@ addflag('pdb', 'InteractiveShell.pdb',
     "Enable auto calling the pdb debugger after every exception.",
     "Disable auto calling the pdb debugger after every exception."
 )
+# pydb flag doesn't do any config, as core.debugger switches on import,
+# which is before parsing.  This just allows the flag to be passed.
+shell_flags.update(dict(
+    pydb = ({},
+        """"Use the third party 'pydb' package as debugger, instead of pdb.
+        Requires that pydb is installed."""
+    )
+))
 addflag('pprint', 'PlainTextFormatter.pprint',
     "Enable auto pretty printing of results.",
     "Disable auto auto pretty printing of results."