The transfer was for an undisclosed fee, previously thought to be worth between £11m and £17m depending on add-ons.
This sees 23-year-old Targett end his 15-year association with Southampton.
”Aston Villa is a club on the up, definitely. Dean, the gaffer, is building something special here,” Targett told Aston Villa’s website after completing his move.
Targett made 21 appearances for Southampton last season, netting his only Saints goal.
Now, he gets the chance to play more first-team football for a Premier League club after Villa beat Derby County in the playoff finals to earn promotion from the Championship.
Villa manager Dean Smith appears excited at the prospect of playing Targett.
“We’re very pleased to have Matt join us,” Smith said. “He’s a player we have been looking at for a considerable amount of time.
”He fits our criteria of being a young player with experience of playing in the Premier League and having the potential to develop further.”