Cooper, 21, joined Whitehaven in March to aid their 2018 season prospects.
He made nine appearances in both League 1 and the Challenge Cup – scoring a total of three tries.
Former St Helens player, now Whitehaven head coach, Carl Forster thought highly of the youngster and expressed his disappointment in losing, saying: “Gutted to lose him but what a player he has been for our club! Wish him all the best!”
The back-rower signed for St Helens from local amateur club Blackbrook Royals. He was part of the unbeaten Academy Championship winning side in 2016 and captained the team.
He then went on to spend the 2017 season on loan at the Sheffield Eagles and then Whitehaven until present.