Long has been a key part of a hugely successful St Helens side
St Helens scrum-half Sean Long has signed a two-year contract with Hull from the start of the 2010 season.
The 32-year-old was given permission to speak to other clubs, with Saints only willing to offer him a one-year deal.
Long has been at St Helens for 12 years, and won every honour in the domestic game with them.
"This move is absolutely fabulous. It really excites me and I really believe it is going to revitalise my career," said Long.
"It has been quite a hectic last couple of days but now everything is sorted I am really buzzing and looking forward to joining Hull."
Long has played 320 games for St Helens, scoring more than 2500 points in the process. But with the likes of Kyle Eastmond coming through the ranks at Knowsley Road, Saints were reluctant to offer Long the two-year deal he was after.
"We fully respect Sean's decision. He has secured a two-year deal on very good terms and we wish him well," said St Helens chairman Eamonn McManus.
"We have a number of very exciting young players who next season will be commanding regular selection and they must be given every opportunity in the long-term interests of the club. St Helens are committed to this policy.
"That said, we must pay full tribute to the incredible achievements of Sean over the last 12 years at the Saints. He is one of the most decorated players in the history of the club and has been pivotal to its pre-eminence in the Super League era.
"His talent has been at the centre of all we have achieved and his personality has always enlivened the place.
"He still has three more pieces of silverware to aim at this season and I know that the St Helens' fans will show their support for this truly great player. He will leave as a personal friend of many of us and as a club legend."
Hull coach Richard Agar added: "Sean is a big name in the game and a big signing for us. I think this will be very important for the club in the way we are looking to move forward.
"We have also done well to have beaten off a number of other clubs."
Former Hull FC scrum-half Craig Murdock told BBC Look North: "I think he's exactly what Hull FC have lacked this season.
"He'll give them that leadership quality and most of all he's got a great kicking game.
"I know he's coming to the end of his career but he's not going to lose his brain and he's certainly not going to lose his kicking game."
Long has enjoyed immense success with Saints after beginning his career with their fierce local rivals Wigan.
He was released by his hometown club after just a handful of games, signing for Widnes and quickly attracting the interest of several bigger clubs.
Long opted to join Saints and has helped them win the Challenge Cup five times and become Super League champions on four occasions.
He has also won the Lance Todd Trophy a record-breaking three times for his man-of-the-match performances in the Challenge Cup finals of 2001, 2004 and 2006, and collected Super League's Man of Steel award in 2000.
Long is no stranger to the eye-catching haircut
For all his undoubted talent and match-winning ability, Long is one of rugby league's more colourful characters with an outgoing personality and frequent hairstyle changes part of a career that has also seen a few twists and turns off the field.
He was suspended for three months by the Rugby Football League in 2004 for betting on a severely weakened St Helens side to lose against Bradford.
In 2006, he left Great Britain's Tri-Nations campaign in Australia amid reports of drunkenness on a flight. Shortly after, he retired from international rugby in order to concentrate on playing for St Helens.
Long's first game since the announcement is for Saints against his future employers on Friday.
The half-back is the second big name that Hull have announced for next season following the confirmation that Australia back row forward Craig Fitzgibbon will be joining them.