The deal was completed today after Arsenal representatives triggered his release clause at La Liga headquarters.
Partey joins Arsenal on a four year deal with an option to extend for another year and will wear the number 18 jersey.
Arsenal head coach, Mikel Arteta said: "We have been watching Thomas for a while, so we’re now delighted to add such a high quality player to our squad. He is a dynamic midfielder with great energy. He brings a lot of experience from a top club that has competed at the highest level in La Liga and the Champions League for several years.
"We’re very impressed with his attitude and his approach to the game. He’s an intelligent footballer and we’re looking forward to him integrating into our system and contributing to the progress we’re building at the moment at the club.”