The 23-year-old's new terms at Twickenham Stoop are due to run until at least 2016.
Marler will make his 100th appearance for Quins on Friday when they travel to Stade Francais in the Amlin Challenge Cup quarter-finals.
"We are obviously delighted that Joe has committed his future to Harlequins. It is incredible to think that at just 23, he will make his 100th senior appearance this Friday," director of rugby Conor O'Shea told the club's official website.
"He is, and will continue to be, a cornerstone in the pack and in the success of this club. Joe is developing into one of the leading loose-heads in international rugby.
"Joe still has so much more to give and like us all, is hungry for more silverware for both club and country."
Marler made his Quins debut in 2008 after graduating from the club's youth academy.