NRL.com reported that conjecture over his future beyond this season has finally been put to bed with the Storm announcing his signing, on a deal understood to be worth around $500,000 (K1.4m) annually, until the end of 2023.
Brisbane had set aside a similar $500,000 figure for Coates, and will now enter the player market to bolster their backline stocks.
NRL.com understands the Broncos have expressed interest in Storm utility Nicho Hynes among several rival clubs, with the utility back impressing at fullback in recent weeks with Ryan Papenhuyzen out injured.