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.

The New Zealand spinner fell two runs short of a century when he was stumped off Jack Leach, who also finished with career-best figures of 7-106.
After bowling Warwickshire out for 324, Somerset then slumped to 9-3 as Keith Barker picked up three early wickets.
Patel's devastating spell saw the hosts bowled out for 110 at Taunton before Warwickshire closed on 20-0.
The 35-year-old's fine innings, which included eight fours and four sixes, took the visitors past an unlikely 300 after they were 120-7 at one stage on day two.
After resuming on 180-7, the Bears' recovery was aided by Laurie Evans who made 73, sharing an eighth-wicket stand of 166 with Patel.
On a third day pitch which had begun to turn, Patel had Jim Allenby caught at short leg, before Peter Trego and Luke Ronchi were both dismissed lbw. Lewis Gregory then could only pick out deep mid-wicket when he looked to launch the spinner over the boundary rope.
Having edged his way to 26 Tom Cooper was bowled to give Patel his third first-class five-wicket haul of the season.
Jamie Overton and Leach were both caught in the slips as Somerset were bowled out in just 34 overs, leaving Alfonso Thomas, in his final innings for the county, stranded on 13.
Varun Chopra and Ian Westwood saw off the final hour before stumps without trouble to leave the Bears requiring another 205 for victory.
Bears all-rounder Jeetan Patel told BBC WM:
"It didn't bother me falling two short of a century because I don't really worry about stats like that. The partnership with Laurie Evans was more important because it got us back in the game.
"The main thing is that it was a good day for the team. After seeing Jack Leach do so well for Somerset it was good for me to get some turn and take wickets. The ball is coming out well for me at the moment.
"We still have a lot of work to do to win the game because 200 more will take some getting on that pitch. Hopefully we can be proactive from the start tomorrow and finish the season with a win."
Somerset fast bowler Alfonso Thomas:
"It was emotional for me walking out to bat in both innings on my last appearance for the club because I have had eight great years here.
"There have been a lot of highlights. Taking four wickets in four balls in a Championship match was great for me personally, but there have been so many memorable times for the team while I have been here and a lot of great one-day occasions at Taunton.
"I still think we can win this game. The pitch is aiding spin, rather than seam, but hopefully I can chip in and we can support Jack Leach. If that happens, it won't be easy for Warwickshire to get 200 more runs."
Harlequins centre Joe Marchant has signed a new and extended contract with the Premiership club.
[NEXT_CONCEPT]
Jeetan Patel took a career-best 7-38 after hitting 98 as Warwickshire fought back against Somerset.