The Gunners have already signed defender Gabriel from Lille for £24m this summer, while Willian arrived from Chelsea on a free transfer.
However, manager Mikel Arteta had been keen to bolster his central midfield options before the window closes, with Arsenal continually linked with Partey as well as Houssem Aouar of Lyon.
On Sunday, Aouar confirmed he would not be leaving his current club, leading to Arsenal turning their attention to Partey.
Arsenal and Atletico had previously been in negotiations for the Ghanaian only for them to fail to come to an agreement, with the Spanish side insisting he would only be sold if his release clause was met.
Now, the release clause has been triggered, with Arsenal paying £45m
Partey currently holds a 85 FC Rating for his performances for Atletico. In the 2019/20 campaign, he made 35 appearances in La Liga for Los Colchoneros.