The former Castleford Tigers star made an instant impact with the Aviva Premiership side following the move from rugby league and has earned himself two caps with England.
Solomona has decided to extend his stay in the 15-man form of the game and Diamond is delighted to see the New Zealand-born flyer pen a new contract at the AJ Bell Stadium.
"He is a world-class finisher who is on the verge of playing for England all the time," he said.
"He's a really great asset to have around and I think it's right we reward him with a long-term contract.
"He has more than justified our decision to bring him over from rugby league a couple of years ago."