Acres impressed in a brief spell at senior level mid-season before ankle and quad injuries ruled him out for all but two VFL matches.
The 18-year-old, who was contracted for 2015, has signed a three-year extension as part of the Saints' 'Future Fest', which has seen key youngsters including Luke Dunstan and Jack Newnes extend their contracts.
Half-back Jimmy Webster committed to the club until the end of 2017 on Monday.
Acres' extension is important for the Saints, who view the young West Australian as a key part of the club's future, comparing his versatility to former St Kilda star Brendon Goddard.
The 190cm utility said moving away from friends and family had initially been difficult, but he was happy at St Kilda.
"This is a big commitment, but the club has been great," Acres told the club's website.
"As I said before, it's just like home.
"We've signed up a lot of the young guys recently, so they are showing they're committed to us and we need to show we're committed to them.
"Everyone can see that a few years down the track, we are going to be a really competitive side."
Young star Jack Billings is expected to announce a contract extension in the coming days, with rookies Mav Weller and Eli Templeton also expected to earn senior list contracts.
