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Partey, 27, has cost Arsenal £45million (€50m) after the club opted to trigger his release clause just 32 minutes before Monday's transfer deadline, and he has signed a long-term contract.
Lucas Torreira has moved in the opposite direction after agreeing to join Atletico on a season-long loan.
Welcome to the Partey! 🥳— Arsenal (@Arsenal) October 5, 2020
👋 @Thomaspartey22#NoThomasNoPartey pic.twitter.com/009Er2kZBK
Ghana international Partey has long been admired by Arsenal and was said to be their primary midfield target, along with Lyon's Houssem Aouar - who has remained in France beyond the deadline.
The Gunners were reported to have made their interest known to Atletico earlier in the window, but Los Colchoneros informed them they were unwilling to negotiate his sale.
Financial constraints were said to be preventing Arsenal from simply triggering his buyout clause, but the Gunners finally stumped up the cash and got their man.
Partey came through Atletico's academy after moving from his native Ghana in 2012.
After loan spells with Real Mallorca and Almeria, he gradually became more prominent at Atleti and has been a regular in the first team for the past three seasons.