Pierre-Emerick Aubameyang opened the scoring on 22 minutes for the hosts, while Renaud Cohade netted a second four minutes after the break,
Rennes' misery was compounded when striker Mevlut Erding managed to get sent off five minutes into injury time.
The result sees St Etienne leapfrog their way up to sixth on 15 points, while Rennes slip back into 10th place two points back.
- Sports & Recreation
- Renaud Cohade
- Mevlut Erding