The 21-year-old, whose new deal will keep him at Craven Cottage until the end of the 2015/16 season, has yet to make a first-team appearance for the Cottagers.
However, he has featured on the Fulham bench recently as back-up to David Stockdale following Maarten Stekelenburg's injury.
Bettinelli was included in the first-team squad after returning from an impressive spell on loan at Accrington Stanley.
He has played 19 games for the Sky Bet League Two side this season and is set to increase that tally as he is back at the Store First Stadium on an emergency loan deal until January 30.
Bettinelli also made 35 appearances in the Conference during a loan spell with Dartford last season.