The 25-year-old has emptied his locker at La Piverdière in anticipation of a transfer, La Provence reports.
Niang joined Rennes for €15m last year following a successful loan spell from Italian side Torino.
Over the past two seasons, the former AC Milan forward has scored 21 goals in 55 Ligue 1 games for Rennes.
His good form has led to links with Jose Mourinho’s Tottenham, but La Provence states that Marseille lead the race.
Marseille began their interest in Niang during the winter, with coach André Villas-Boas giving his approval.
Villas-Boas appreciates Niang’s versatility and would like to add him to the OM squad ahead of their Champions League campaign next season.
Rennes are hopeful of profiting on the Seneral striker following his good season, with an €18m fee mooted.