Sport says West Ham have agreed a €15m fee with Barca for the Brazilian defender.
Marlon had joined Nice on a two-season loan last year, though that was cut short last month. The OGCN deal had included a €6m permanent option.
West Ham and Barca are now putting the finishing touches on a deal for Marlon, 22.
The stopper signed for Barca two years ago from Fluminense.