Raisuqe had played seven times this season for Castres, his last appearance in the 52-6 thrashing by Toulouse on April 27.
Raisuqe, who was fired by Stade Francais in 2017 for gross misconduct, was set to play for tier-two side Brive for the next two seasons.
“He was a shining light of a boy on and off the pitch,” said Castres owner Pierre-Yves Revol.
“He was a pillar of the Fijian community which we have at the club (there are three other Fijians) and to whom we are very attached.
“It is difficult also at this time not to evoke the faith of all the players and of Josaia.
“That, aside from the grief, will help them in this very difficult time for everyone.”
Castres, who are fifth in the table, cancelled training for the day and Saturday’s match with Clermont has been postponed, the French National Rugby League (LNR) announced.
The LNR expressed their “extreme sadness” at the “tragic death” adding homage would be paid to him at all Top 14 and second tier matches this weekend.