Ablett's decision to sign with the AFL’s 17th club was revealed by Suns CEO Travis Auld and senior coach Guy McKenna at a packed media conference at Melbourne's Crown Casino on Wednesday.
Ablett said his decision had been "incredibly difficult" to make, but he felt it was time for "a fresh new challenge".
He said that money was not the main motivating factor in his decision.
"I gave my all to Geelong and will always be grateful to a club that has been such a big part of my life.
"While the talk will be about money, the decision was not solely about money. The challenge ... of the football field is much more exciting to me."
Ablett said he made his final decision on Tuesday after heading to Queensland for a break, and that he informed Geelong on Wednesday morning.
He will still attend Thursday night's Geelong best and fairest presentation at which he is expected to win the main award.
“It’s an opportunity to be involved in something from the beginning - to help shape and develop a young talented playing group - one that has the potential to be very successful,” Ablett said.
“I am very excited by the vision of the Gold Coast Suns, the support the team has already achieved and the quality of the group of people involved. I’m looking forward to the next stage of my career.”
Ablett said there had not been extensive discussions about the Suns' captaincy, but it would be 'fantastic' to be named the club's inaugual skipper.
Auld said Ablett was "widely recognised as the competition's best player" and the biggest marquee player in the AFL.
He said Ablett had taken on a huge personal challenge in joining the new club, and his signing was "the best endorsement" of where the club stood.
McKenna said Ablett's competitiveness, work rate, team ethos, training and rehabilitation detail would provide outstanding leadership to the Suns' young group.
“He brings a level of professionalism from a premiership team and a team that has dominated the competition for much of the past four years which will be invaluable across all aspects of our club," he asaid..
“We are not just about building a football club. We are about building a successful football club and attracting a successful player such as Gary Ablett to the Gold Coast Suns was an important part of our plan.”
The 26-year-old, who won the 2009 Brownlow Medal and was runner-up this season, has signed with Gold Coast for five years.
Geelong recruited Ablett under the father-son rule in 2001. Since making his debut in 2002, he has played 192 games and booted 262 goals.
His brilliant form during the past four seasons has helped the Cats dominate the competition, winning premierships in 2007 and ’09.
Details of the compensation provided to the Cats will be announced later.
The news of Ablett’s defection is a bitter blow for the Cats, as it comes just a day after senior coach Mark Thompson told the club he was reconsidering his position.
Thompson was widely tipped to quit Geelong on Tuesday and take up the offer of a senior assistant coaching role at Essendon - the club he previously captained - under its new boss James Hird.
But Cats chief executive Brian Cook later revealed that Thompson, who has a year to run on his contract, was suffering from exhaustion.
Cook insisted that he does not believe the dual premiership coach will join the Bombers.
Geelong has asked Thompson to decide by the weekend whether he can commit to the club for another season.
