The winger, who played for Metz and Rennes before joining the northwest London club, will add depth to Marseille's squad. The French side did not specify the length of his contract, although according to several media reports, he has signed for five years and for around €13million.
Sarr will wear the number 23 shirt.
After a successful first season with Metz in 2016, the Saint-Louis-born player signed for Stade Rennais the following year, scoring 18 goals and providing 16 assists in 77 appearances over two seasons - enough to attract the interest of Watford in 2019.
With the Hornets, with whom he most recently played in the Championship, he racked up 34 goals and 24 assists in 131 appearances.
At the age of 25, Sarr also lifted the African Cup of Nations in 2022, having already won 54 caps for his national side. This is the second major signing in the space of a few days for Marseille, who, after signing Gabonese striker Pierre-Emerick Aubameyang (34) from Chelsea on Friday, have remained very active in the summer transfer market.
These two players, known for their pace, will provide new coach Marcelino with greater depth and should fit into a 4-4-2 formation that the new Spanish coach is keen to see.
The arrival of these two attacking recruits is likely to settle the question of the future of Chilean Alexis Sanchez (34) and a firm favourite at the Velodrome, but whose contract expired at the end of June and who has yet to show any inclination to extend it.