The 27-year-old has made 22 appearances this term for the Broncos, who recently suffered relegation from Super League.
"I'm excited by how the Tigers are playing this season," Cook told Castleford's official website.
"I already know a few of the boys and I've heard nothing but good things about the place, the players and the coaches.
"To be part of a club that is challenging for silverware and to be part of what they have built here will really be something special."
Castleford return to Super League action on Thursday when they make the trip to Hull FC.