Marseille, third in the French league standings but level on 38 points with leaders Paris St Germain, did not disclose the fee no the length of the 29-year-old Kadir's deal.
A source close to the club said the attacking midfielder, whose contract at Valenciennes was due to end in June, has signed for three and a half years.
Kadir, who has 15 Algerian caps, has been called up for the African Nations Cup, which takes place in South Africa from Jan. 19 to Feb. 10.
"The recruit will have to leave Marseille for a short while in January but he took part in his first training session with his teammates this Wednesday," Marseille said on their website (www.om.net).
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.
($1 = 0.7585 euros) (Reporting by Gregory Blachier; Editing by Ed Osmond)