Jooste, who joined Leicester Tigers from Bourgoin this season, goes straight into the Castres squad for this weekend’s league match against Toulon.
The South African has recovered from a shoulder injury suffered in his only appearance for Tigers, in an Aviva Premiership ‘A’ League fixture at Gloucester.
Tigers director of rugby Richard Cockerill said: “Wessel has not had much opportunity to play for us, but this is a good chance for him to go to and play.
“It has been a bit unfortunate from start to finish, he had a nasty injury in his shoulder in his first game with us but now he has an opportunity to go and play because Castres have some injury issues.
“It is the right thing to do to allow him to go and get an opportunity.”