"With 12 matches remaining this season, a top-10 finish is still possible and would represent another good season for the club as we aim to play in the Barclays Premier League for a 13th consecutive campaign.
"I will always give 100 per cent for the chairman and the club, and I look forward to another season playing at Craven Cottage, which really is a special ground."
Fulham have tied down Duff amid some concern they could lose a number of players in the summer.
Veteran goalkeeper Mark Schwarzer could, however, soon join him in staying at the club after revealing earlier this month that he has begun talks over a new deal.
Boss Martin Jol has previously also stated his desire to pin down on-loan defender Sascha Riether to a permanent deal.