Smith has been a revelation for Quins after bursting onto the scene this season, prompting England coach Eddie Jones to call him up as an 'apprentice'.
The promising 18 year-old has been rewarded with a first professional deal with the Premiership club.
"I'm delighted to have secured Marcus' long-term future at Harlequins," Quins director of rugby John Kingston said. "Marcus is an intelligent and humble young lad who is very focused on his rugby. He has the talent and abilities to take this Quins side forward onto the next stage of its development.
"He is the type of player and person that perfectly embodies the Harlequins spirit and culture we are building at The Stoop."
"I am extremely proud to share the news of my contract extension with Harlequins,
"Harlequins is a fantastic club and they have looked after me extremely well since I joined three years ago. The club is incredibly ambitious and combined with John Kingston's long-term vision, it made the decision much easier knowing the faith and guidance he has shown me.
"As a player you want to be playing in the big games and hopefully if I can keep improving I will be involved in more and more of them and help the club progress both domestically and in Europe."