WA officials are disappointed at losing the 23-year-old left-arm spinner, but NSW coach Trevor Bayliss said Casson's signature ended a two-year recruiting effort.
He will vie with Test spinner Stuart MacGill and former Queensland off-spinner Nathan Hauritz for places in the NSW line-up in next season's Sheffield Shield, although Hauritz is yet to secure a NSW contract.
"The prospect of having left and right arm wrist spinners is exciting," Bayliss said Monday.
"Our spin bowling stocks at that level haven't been all that deep, but with Beau coming and quite possibly Nathan, it creates some competition."
Bayliss said MacGill backed the move and sees Casson as his eventual successor.
"One of the reasons we went after Beau is the SCG wicket is conducive to wrist-spin bowling," Bayliss said.
"We think Beau has got that potential to go further than just state cricket. He's the right age."