The 23-year-old former Salford centre will link up with Manu at Langtree Park under new head coach Nathan Brown later this year after signing a two-year contract.
"Jordan is one of the most exciting talents in the game at the moment and we are delighted to secure his services for the next two seasons," said Saints chairman Eamonn McManus.
"Not only does he possess good feet and a great turn of pace, but he's matured into an excellent defender too.
"At 23, he is still developing and will no doubt do so further under the tutelage of Nathan Brown and his staff."
A former England academy international and captain, Turner joined Hull from Salford in 2010.
He will be a straight replacement for Michael Shenton, who is returning to Castleford.