Purcell has played under the Hull coach, Peter Walsh, at his Australian club and said that was a major factor behind his decision to move to The Boulevard.
"You can have the best contract in the world but if you're not happy and don't get on with the coach it can become a nightmare," said Purcell, who last season appeared 19 times in the first grade side for the club, which has now been merged with St George.
"Knowing Peter was crucial when it came to deciding and I also know some of the Sharks players from my time at Illawarra," he added.
Purcell's arrival ends any possibility of the New Zealander, Brad Hepi, returning to Hull. He is on the market after walking out of his rugby union stint with London Scottish but is not wanted back at The Boulevard and could now be destined for newly-promoted Wakefield Trinity.
Purcell is Hull's fourth close- season signing and the club also intends to announce two more new recruits - one of them an established international - tomorrow morning.
Halifax have secured the services of their Wales and Great Britain prop, Kelvin Skerrett, for another season.
"Kelvin was one of our impact players last season and his influence on the youngsters at Halifax is invaluable," said David Hobbs, the football manager at the club, which surprised the game by rising to third place during a successful Super League season.
"Securing his services for next season was a major goal for our coach, John Pendlebury, and should reassure our supporters that our pack next season will be as strong and fearsome as ever."
The retention of Skerrett is an important step in the right direction for Halifax, who released his front-row partner and club captain, Karl Harrison, at the end of last season. Their efforts to sign another Test prop, Sheffield Eagles' Paul Broadbent, foundered on the player's demand for a club car.
Speculation is now rife that Broadbent, the Sheffield captain capped eight times by Great Britain, could be one of the two British signings to be unveiled at Hull tomorrow.Reuse content