The 22-year-old, who has played once for England, was suspended in January 2022 by Manchester United following allegations of attempted rape and sexual assault but prosecutors dropped charges in February 2023. He spent last season on loan at Getafe in La Liga.
"Olympique de Marseille are delighted to announce the signing of Mason Greenwood from Manchester United. The 22-year-old striker has signed for the club following a successful medical," said the French club on their website.
Greenwood made his debut for Manchester United aged 17 years and 156 days in the Champions League in 2019 and his Premier League debut four days later. He made his debut for England in September 2020.
The forward made 129 appearances and scored 35 goals for United but had not played for the Premier League club since January 2022.
"We wish Mason all the best in his future career," said United on their website.
Before the transfer became official, Marseille's left-wing mayor Benoit Payan told French media: "I don't want my club to be covered by shame."
Marseille made no mention of the allegations in their statement. They called Greenwood "a precocious talent" and an "English nugget". The club said their new striker will wear the number 10 shirt but did not specify the length of his contract or mention any fee to Manchester United.
Greenwood scored 10 goals in 36 games for Getafe last season.
"I did not do the things I was accused of, and in February I was cleared of all charges," he said when he joined the Spanish club.
"However, I fully accept I made mistakes in my relationship."
His arrival in Marseille comes the day after the official departure of Pierre-Emerick Aubameyang, the Gabon international who was Marseille's top scorer last season.
