Only six of Armstrong’s 11 appearances for Wednesday were starts, but Aberdeen no doubt see him as part of their Conference League plans considering his signing coincided with the closure of Uefa’s signing deadline.
“I hope I can add something to the team and help us achieve something special together,” he told the club’s website.
“I’ve not been back to the city for a long time, so it’s nice to come back full circle in a football sense.”
Armstrong had joined Wednesday from Vancouver Whitecaps, where he was restricted to four starts and five substitute appearances after his six-year spell with Southampton ended last September.
Thelin, though, had “been tracking Stuart for some time” before making the former Celtic midfielder his 13th summer addition.
“We are so pleased to be able to add a player of his talent and experience to our group,” Thelin said.
“Stuart has had an impressive career at the top level of the game and we hope he can act as another leader in our squad.
“With such a busy schedule coming up, we want to drive competition in all areas of the park who can bring different attributes to how we play.
“We also have several very talented young prospects and having a core of experienced players from Scotland to help them develop has always been part of our strategy.”