Victorian product Amartey had been keen to add to his nine seasons at the Swans, but changed his tune when Carlton upped their offer to a reported five-year deal.
Sydney were unable to match the offer and confirmed Amartey's decision on Wednesday ahead of the upcoming free agency period in October.
Amartey was left as an emergency during finals after failing to convince coach Dean Cox following a partial Achilles tear.
The 27-year-old had been sidelined since round 16, with Hayden McLean impressing in his absence.
His Achilles tear was the latest setback for the injury-prone forward, who has played 76 games since making his debut in 2020.
Amartey played a career-best 25 games - which included a nine-goal haul against Adelaide - in 2024, but has struggled to replicate that form and played only eight matches the following season.
He kicked 28 goals across 15 games this year, with Charlie Curnow leading the league with 75 goals in his first season as a Swan.
At the Blues, Amartey will reunite with former Swans teammates Will Hayward and Ollie Florent after the pair were ruthlessly traded last season for Curnow.
Amartey leaves the Swans as the club mulls the playing futures of suspended group Isaac Heeney, Chad Warner, Nick Blakey, James Jordon and Riley Bice.
All five are contracted for next year, with Blakey signed to the club until 2031.
Warner and Jordon come off contract at the end of 2027, while Heeney and Bice are signed until the end of 2028.
Blakey is reportedly seeking a trade to Victorian rivals, but football boss Leon Cameron has dismissed those rumblings as "speculation".
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