The forward scored 10 league goals in 29 appearances last campaign for FC Koln as he helped the 2.Bundesliga side to the title in May. The 21-year-old is a giant at 6ft 4ins and should help add firepower to a Saints attack, hoping to propel the club back up to the Premier League.
Speaking to the club website, Downs said he wants to do exactly that and revealed that he believes Southampton belong in England’s top division.
"Everybody plays football to win titles. To win my first title at 20 last year was a great feeling, and I hope to repeat it again this year.
"I think Southampton is a big club that should be in the Premier League, and that's my goal – to help the team as much as I can to bring the club back to the Premier League."
Technical director, Johannes Spors opened up the deal for the USA international and stated that Downs is exactly the type of player the club needs to bounce back next season in what will be another tough Championship campaign.
“Damion is a player we have been monitoring for some time and possesses excellent physical qualities and the ability to disrupt defences.
“Strengthening our attacking options was one of our top priorities this summer, and Damion is a player who will add real quality to the group.
"Despite his relatively young age he is already a senior international, so we are excited to see him thrive in the environment here at Southampton.”