The deal, which has yet to be officially lodged with the AFL, also involves a swap of later picks, with the Saints receiving picks No.26 and 47 and the Suns receiving 37 and 57.
The Saints have been pursuing Hickey, 21, throughout this month's Gillette AFL Trade Period as they seek to provide more support for their No.1 ruckman Ben McEvoy.
St Kilda's head of football Chris Pelchen said the Saints were delighted to secure Hickey.
"We are extremely pleased to have acquired Tom. At only 21 years of age, we feel Tom still has significant football development ahead of him," Pelchen said.
"Along with Ben McEvoy, [Hickey] will provide the Saints with a strong ruck combination for the future. It has been widely acknowledged that St Kilda was seeking to complement its ruck stocks and securing Tom was identified as a priority during the 2012 Gillette AFL Trade Period."
The Suns were reluctant to part with the ruckman who they took as a local talent access selection in 2010, reportedly refusing the Saints' earlier offer of pick No.25 and stressing they would only accept a first-round pick.
The Saints traded their original first-round pick, No.12, for Greater Western Sydney pre-listed player Tom Lee (Claremont) but subsequently received pick No.13 as free agency compensation for losing Brendon Goddard to Essendon.
Suns football manager Marcus Ashcroft said the deal significantly improved the club's position for November's NAB AFL Draft on the Gold Coast.
"We were really keen to gain a selection back inside the top 15 since exchanging our first round draft selection [No.2 overall] for the extremely talented Jack Martin through the trade incentive draft," Ashcroft said.
"Our ability to add pick 13 to our selections in the upcoming draft means we will have the ability to draft another elite young player to our club next month."
Hickey played 12 games in the Suns' first two AFL seasons, including 10 this year.
Although he was recruited locally from Morningside, Hickey requested a trade to a Victorian club at the end of this season to move closer to his girlfriend.
He nominated the Saints as his preferred club, but North Melbourne and Essendon also expressed interest in him.
Nick Bowen is a reporter with AFL.com.au. Follow him on Twitter: @AFL_Nick
