The 27-year-old signed a long-term deal with the Gunners, having paid his 50m euros (£45.3m) release clause.
Partey made 35 league appearances and scored three goals for Diego Simeone's side as they finished third in La Liga last season.
"Dear Atletico fans, today I only have words of gratitude with you. For several years now, Atlético de Madrid has been my home and a part of this family will always be with me. I want to thank the club for the trust placed in me from day one, but above all, I want to thank you, this wonderful hobby that I accepted from the first moment and that has shown that it is both in good times and in bad times and you never truly stop believing. Atlético de Madrid is not just a club, it is a family. Thank you ♥" a post translated from Spanish on Partey’s official Instagram page reads.
https://www.instagram.com/p/CF_ucBwgI5m/?utm_source=ig_embedThe Ghanaian midfielder played 188 times for Atletico after joining the club in 2011.
He won the 2017-18 Europa League and 2018 Super Cup with Atletico and signed a new deal at the Wanda Metropolitano in 2018, until the end of June 2023.
With the transfer window now shut, attention will switch from club to international football, with Partey flying to Turkey to link up with the Ghana squad. The Black Stars will take on Mali on Friday 9 October and Qatar on Monday 12 October in international friendlies.