Romain Alessandrini finished a Julien Feret cross in the 34th minute to give Rennes the lead.
They doubled their lead after the hour mark through Mevlut Erding's spectacular finish from a Jean II Makoun cross.
Spurs and QPR target Yann M'Vila handled in the box to give woeful Bastia a penalty four minutes from time, but Anthony Modeste struck it wide after slipping in the execution of it.
Feret then hit the post for Rennes, but the visitors were comfortable winners.
Rennes are seven points behind leaders Lyon while Bastia stay 13th.