The 27-year-old has signed a long-term deal with the Gunners and will be wearing the number 18 shirt, which had been vacated by Nacho Monreal the previous season.
Atletico Madrid released a statement on Monday confirming Partey’s departure.
A statement read: “LaLiga informed Atlético de Madrid at 11.28pm on Monday that Arsenal representatives have paid Thomas Partey’s release clause at the sports association’s headquarters.
“The player has therefore unilaterally terminated his contract with our club, which ran until June 30, 2023.”
Partey had long been linked with a move to the Emirates Stadium since the beginning of this summer’s transfer window.
Arsenal head coach Mikel Arteta admitted that the Londoners had been mulling over the possibility of signing Partey for quite sometime and are now delighted to add such a quality player to their ranks.
"We have been watching Thomas for a while, so we're now delighted to add such a high quality player to our squad,” the Spaniard said.
"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."
Partey graduated from the Atletico Madrid academy and was loaned out to Mallorca and Almeria to gain experience.
The Krobo Odumase-born star established himself as one of the key players under Diego Simeone since making his debut in November 2015 in a 1-0 victory over Espanyol.
Partey made 188 appearances for the Rojiblancos scoring 18 times and providing 12 assists.
He also won the Europa League and the UEFA Super Cup during his time in Madrid.
