The 23-year-old has agreed a four-year contract at St Mary's, with Saints beating Middlesbrough and several other suitors to the deal.
Dobbin arrives on the back of a breakout campaign on loan at Preston North End, where he plundered 11 goals and laid on eight more across 41 Championship outings. Remarkably, Aston Villa never handed him a senior debut following his £10m arrival from Everton in 2024, instead loaning him to West Bromwich Albion, Derby County and Norwich City before his year at Deepdale.
"Obviously I played against the team twice last season and you can see the quality they have and I only think it's going to improve my game massively and it's that next step for me," Dobbin told club media.
Southampton technical director Johannes Spors is convinced that the forward's pace and adaptability will add a sharper cutting edge to the club's attack.
"He is already, at a young age, a proven player in the Championship who is always a threat to defences with his speed and ability to stretch teams," Spors told the club website.
"His versatility gives us greater flexibility in our forward line. We look forward to seeing him thrive with us not only next season, but in the years ahead."
Comfortable operating through the middle or on either flank, Dobbin gives manager Tonda Eckert plenty of attacking options behind the striker, and the new signing has made no secret of exactly where on the pitch he feels most dangerous.
"I like to be as positive as possible," he said. "I think I can play anywhere in the front line and I think that will help me get in the team, being dangerous in as many positions as possible.
"As soon as I get the ball I want to look forward, I want to drive. I want to create and score goals."
His 19 goal involvements last season marked him out as one of the division's most productive attackers, and the new arrival believes the move can carry him towards the top flight, describing Premier League football as the ultimate goal as Southampton gear up for another promotion push.