Saints agreed to pay pounds 300,000 in cash and part with the pounds 70,000-rated Welsh international forward Chris Morley to Warrington to complete the transfer.
St Helens already possess the costliest back in Paul Newlove - he joined them in a pounds 500,000 cash-plus-players deal two years ago - and exceeded the pounds 250,000 Leeds paid Wigan for Ellery Hanley in 1991 to land Sculthorpe.
The Saints coach, Shaun McRae, said: "At the moment, for his age, you have got to say he is one of the best players in the world."
Oldham have paid an undisclosed fee to Rochdale to sign their former captain Neil Flanagan, 27, a scrum-half who also played for Huddersfield. David Mycoe and Alex Thompson have left Sheffield after failing to agree terms with the club.Reuse content