Marchant, 20, has made 37 appearances for Quins since making his senior debut in May 2015, scoring 11 tries.
"I'm delighted to have re-signed with Harlequins," he said. "This is such a great club and I have loved my time here. It was an easy decision to stay."
Marchant had been due to tour Argentina with England last month but was ruled out with a toe injury.
Harlequins centre Joe Marchant has signed a new and extended contract with the Premiership club.