In the highest-profile deal of deadline day, Partey has joined the north London club and signed a four-year deal with the Gunners.
Partey was among the club’s primary targets in the transfer window, with Arsenal already signing Brazilians Willian and Gabriel earlier this summer.
The midfielder will wear the number 18 shirt at his new club, with the previous owners of that number having mixed careers in a Gunners shirt.
The last player to wear the number was Spaniard Nacho Monreal, who had a great career with the Gunners before exiting last season.
Prior to Monreal, French defenders had a common allegiance towards the 18 shirt at Arsenal. Previous owners include Sebastien Squillaci, Mikael Silvestre, Pascal Cygan and Giles Grimandi. Steve Morrow and David Hiller have also worn that number.
In past four seasons, Partey has worn the number 5 shirt at Atletico. Prior to that, he held the 22 shirt, which he also wore at previous club Almeria.
The number 5 was not available at the Emirates, with Sokratis Papastathopoulos retaining that number having not left north London as had been expected during this transfer window.