Steele will have surgery on Monday after he left the ground in the third quarter of Saturday night's 51-point win over the Western Bulldogs at Marvel Stadium.
He played out the game, but had limited range in his arm.
It adds to the Saints' lengthy casualty list and means their captain will be sidelined for next Saturday night's club 150th anniversary match against Essendon at the MCG.
"Jack's as courageous as they come and to play out the game with a broken collarbone was a fantastic effort," St Kilda football boss David Misson said in a club statement.
The 27-year-old ruptured the AC joint in the same shoulder last year, requiring surgery and his injury added a sour note to a night where the Saints went 2-0 under Ross Lyon.
"It's not feeling great but we'll just have a few scans and see how it is," Steele told the Seven Network.
Lyon was otherwise confident the Saints had avoided injuries, but said his players needed to get used to the hard surface at Marvel Stadium again.
"There's a couple who are a little bit sore but we're optimistic they'll come through," he said.
Lyon said St Kilda are looking forward to marking their anniversary against the Bombers.
"We'll enjoy tonight, seven-day break," Lyon said.
"Go watch Essendon tomorrow, we're gonna have the 150th anniversary at the MCG.
"It's a really big week for the club and a big night but as a playing group we'll try and honour that with the effort that it deserves, show the esteem we hold this club in.
"But in saying that, we can't get caught up and distracted. We've got to be unwavering in our commitment to how we prepare."
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