Wardlaw booted 22 goals last season in Brisbane's run to the grand final but has joined fellow guns Emily Bates and Greta Bodey (both Hawthorn) in departing the Lions for Victoria.
St Kilda gave up picks No.6, No.14, No.31 and defender Clara Fitzpatrick (Gold Coast) in a deal that delivered Wardlaw and Suns defender Serene Watson.
Wardlaw will spearhead St Kilda's attack with Caitlin Greiser and Kate Shierlaw departing for Richmond and North Melbourne respectively.
"I loved my time with Brisbane and will forever have a special bond with the girls I played with there," Wardlaw said.
"But to take my footy to the next level I wanted to challenge myself, move down to Melbourne and join a club with a future I really believe in, and I think I've found that at the Saints.
"I can't wait to get stuck into training, meet more of my teammates and do everything I can to help achieve success with this team."
The Saints, who previously traded for Collingwood pair Steph Chiocci and Jaimee Lambert, expected Wardlaw and Watson to immediately have an impact.
"Jesse is a premiership player and current All-Australian," St Kilda AFLW boss Tessie McManus said.
"Her ability in front of goal is so impressive and to think that she's only 23-years-old should excite Saints fans.
"We strongly believe her best football is ahead of her and we can't wait to have her playing with us.
"Serene is a tall, rebounding defender who really embodies the way we want to play footy here at the Saints."
Along with Fitzpatrick, the Suns also landed former Magpie Jordan Membrey, with Collingwood receiving pick No.14 in exchange, plus former North Melbourne forward Ella Maurer.
Brisbane received former Suns midfielder Ellie Hampson and North Melbourne's pick No.40 in the deal.
North Melbourne received ex-Lions defender Lulu Pullar and pick No.31.
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