Aribo joined Rangers from Charlton Athletic in 2019, and has since then developed himself as one of the best midfielders in Europe.
According to many reports, Joe played the most minutes by any player in the world for club and country with over 70 caps.
Southampton manager Ralph Hasenhüttl said: “Joe is a player with many qualities to his game, and we are very excited to welcome him to the squad in time for the pre-season trip to Austria.
“We believe Joe can bring energy and creativity to our side. Feeling comfortable in so many different positions is very helpful for a manager and shows he has a high level of football intelligence, with the ability to take on new information quickly.
“He has played a lot of games already in his career, at a very high level, and his performances in the Europa League prove he has the quality to play in the Premier League. This is a big ambition for him, and he is hungry to show everybody he can do it.”
Joe Aribo said: “It’s an amazing club with a lot of history here, and it’s a good plan for what’s to come next. I’m excited about the journey.
“The Premier League is the best in the world. I’ve been watching it since I was a young boy, it’s always been a goal for me to play here and show what I can do on this stage.
“It’s an amazing feeling, a dream come true. This is where I’ve wanted to be since I was a child growing up playing, so to say that I’m here now is exciting.”
Joe Aribo on his Instagram page bid his farewell to Rangers:
“It’s been an amazing journey being part of this special club. I hold @rangersfc very close to my heart, as they allowed me to showcase my talent on a bigger stage. Playing at Ibrox was a dream & I’m glad I got to experience it. I’m happy I was able to play a part in the historic 55 league title-winning season. I’d like to thank everyone associated with this great club for all you’ve done for me during my time here, & you’ve always made me feel welcome & I’ve always had your full support. I’ve developed into an even better player, credit to the supporters who always had my back and stayed patient during the tough times. We created a lot of memories together, memories that I will carry with me throughout the rest of my career. I wish the club nothing but the best for the future & I’ll always be checking in with Joe Aribo.”