There is also an option included to extend his deal by a further year.
The club announced the Frenchman had put pen to paper on fresh terms on Thursday. In a club statement, Martial gave his reaction to the deal:
"I am loving my time at this club. From the day I joined I have been made to feel part of the United family and I have been incredibly humbled and overwhelmed by the warmth and love of our fans, who continue to amaze me with their support.
"I would like to thank [interim manager] Ole [Gunnar Solskjaer] and his coaching staff for their belief in me and for helping me to take my game to the next level. This club is all about winning trophies and I am sure the next piece of silverware is not that far away."
Martial was in the final 18 months of his previous contract, perhaps having been reluctant to agree to a new deal because of his rocky relationship with former manager Jose Mourinho.
Mourinho fined Martial in the summer following the forward's decision to leave United's pre-season tour of the United States to attend the birth of his child, and the manager had reportedly wanted to sell him, per Kris Voakes of Goal.
The 23-year-old arrived at Old Trafford as a teenager in 2015. He has since made 161 appearances, scoring 46 goals and producing 27 assists.
He netted his 10th of the 2018/19 season in United's 3-1 FA Cup win over Arsenal last Friday:
Martial has made a habit of scoring in some of the Red Devils' biggest matches this season, per football analyst Dave O'Brien:
Although he has spent some time as a centre-forward, Martial has typically been used on the left wing during his time with United, where he has been an electric presence on the ball.
The France international has pace to burn and the dribbling skills to accompany it, making him very difficult to defend against, and he's a composed finisher in front of goal too.
His development perhaps hasn't gone quite as expected, given the difficulties United had in attack under Mourinho and his predecessor Louis van Gaal, but it remains clear Martial has the potential to be a top talent.
With his future secured and United adopting a more positive approach under Solskjaer—under whom they've scored 24 goals in nine games—Martial could be set to go from strength to strength at Old Trafford.