Moussa Djenepo has become Southampton's first signing under manager Ralph Hasenhuttl.
The winger joins from Belgian club Standard Liege, where he scored eight league goals last term, for a reported £14 million ($17.8m) fee.
Djenepo, 20, is a Mali international and part of their squad for the upcoming Africa Cup of Nations.
"It is something I am very proud of," he told Southampton's website. "It's a dream come true. Southampton is a great club and I am very happy.
"It's a good club that is progressing, it has great facilities, and it's a place that can help me to really kick on and progress.
"I am looking forward to it and I want to get to know my new family. I can't wait to be around my new team-mates and work hard alongside them."
Hasenhuttl feels Djenepo is the right profile to thrive at Southampton, who are away to Burnley on the opening weekend of the new Premier League season.
"We are very happy to welcome Moussa as the first new player to join our team ahead of the upcoming season," he told Southampton's website.
"He is an exciting player, with tremendous pace and good finishing abilities, who we have watched closely for some time.
"We have been impressed with how he has adapted to football in Europe and the impact he has had in many games, and we had an interest in him as recently as January, so I am pleased we are able to bring him into the club.
"He combines his quality with a good mentality and he is the right profile of player that we want to sign – still in the early part of his career, already with a lot of talent, but also with a great amount of potential that I believe we can help him to fulfil.
"I think he will be an excellent fit for our style of play and shape, and these are the important characteristics we must look for.
"It is about getting this right sort of quality, not quantity – we must maximise the use of this window by spending our money in the right way and on the right type of player, and this is a very strong start."