GMX said the service ensures its 11 million users web users have fewer IDs and passwords to remember.
The webmail service will also allow users to see updates to their Facebook News Feed, without navigating away from their inbox, as well as receiving Facebook emails direct to their GMX account.
Facebook messages are also personalised with the person's profile picture and their latest status.
"Email and connecting with friends through the social web are two of the most frequently used tools online," said Eva Heil, managing director, GMX.
"Based on feedback from our GMX users, we know that there is a longing to link these two separate'online worlds'. Facebook Connect lets us bring them one step closer to finally closing this gap."
See also: 33% of Brits use personal email at work