The 32-year-old decided to part ways with the Saints when his contract ended in the summer, even though they had achieved promotion to the Premier League.
Now Armstrong has found a new team, having signed for Major League Soccer outfit Vancouver Whitecaps.
The Whitecaps’ sporting director Axel Schuster said: “We are very excited to welcome Stuart and his family to Vancouver.
"He is a player who has excelled for many years at the highest levels in the United Kingdom, as well as internationally with Scotland.
"He brings experience, tactical awareness, positional flexibility, and a very high work rate.
"In addition, his passing range and elite chance creation from the midfield position will add another dimension to our team’s overall attack."