Marseille will pay a reported €500,000 fee for the 29-year-old, whose contract was set to expire in the summer.
He has reportedly signed a three-and-a-half-year contract.
Kadir could make his debut on Sunday as l'OM return from the winter break with a Coupe de France match at home to Ligue 2 outfit Guingamp.
He will then travel to South Africa as he has been picked to play for Algeria at the African Nations Cup.
Kadir, who has played 82 Ligue 1 games, has been key for sixth-placed Valenciennes this season, scoring six goals and setting up four in 18 appearances.
Marseille are third in Ligue 1 on goal difference as Paris Saint-Germain and Lyon above them also have 38 points.