The 25-year-old spent the last three years at Ibrox where he scored 26 goals in 149 appearances.
Aribo not only helped Rangers to their first Scottish Premiership title in a decade, but also claimed the Scottish Cup and was a runner-up in last season's Europa League.
The Nigeria international has 20 international caps to his name and has built a reputation for his versatility on the field.
Upon signing, Aribo told Southampton's website: "It's an amazing club with a lot of history here, and it's a good plan with what's to come next. I'm really excited for the journey.
"The Premier League is the best league 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 really exciting."
Southampton manager Ralph Hasenhuttl added: "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."