The Kenyan international defender has signed a four-year contract with Mauricio Pochettino's side.
He told the club's official website: "It feels great and I am happy to be here. Finally it has happened and we've come to the end [of negotiations], so I am delighted. There were other options, but Southampton is a club with ambitions so I chose to came here.
"Southampton is a good club and it has good players, so I just want to train hard and work hard to be a part of the starting XI. We'll see where that will take me. I'm very grateful to be here and I'm looking forward to playing in the Premier League.
"I've seen that the stadium is great and the dressing room is looking good, and I know some of the players. I hope they will help me and I think I will get along with the others, so it will be good. I am happy to be a part of Southampton's squad. It means a lot because because it has been my dream to play in the Premier League and now I am here."
The 21-year-old will now join his new teammates tomorrow ahead of a pre-season tour of Europe, which includes matches in Spain and Austria.