The Ghanian joined on deadline day after Arsenal activated his release clause, and Arteta is pleased to sign Partey after a summer-long pursuit of the midfielder.
“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," said Arteta.
"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."
Technical director Edu added: "Thomas has all the right attributes to be a top player for Arsenal. He is a leader on and off the pitch and he is exactly the kind of player and person we want at the club. We already feel like we know Thomas very well based on the work we have done analysing his performances closely in recent times.
"With his all-round game and positive aggressive style, he will be a brilliant addition to our squad. We’ve made a strong start to the season and we want to continue to build on this with Thomas now part of our club."