Rennes announced the surprise move on its website on Thursday afternoon. The 25-year-old Erasmus signed on a two and a half year deal.
Erasmus scored 13 goals in 51 matches for the Buccaneers. He has eight Bafana Bafana caps and has scored one goal for the national team.
The move means he will not play in the Soweto Derby this weekend, between Pirates and Kaizer Chiefs.
