The midfielder said it was “great feeling” when his agent informed him Gunners had activated his release clause.
Partey joined Mikel Arteta’s side from Atletico Madrid on transfer deadline day in a deal worth £45 million.
The 27-year-old will wear the number 18 shirt at Arsenal, and will reportedly earn £250,000 per week.
He was officially unveiled on Tuesday as he posed for photos in Arsenal’s home and away kits.
“Until the last moment, I was not aware. My agent told me the move was going to happen, I was so happy,” Partey told Arsenal TV.
“I could not even sleep that night, it was a great feeling.”
Meanwhile, Partey has overtaken Michael Essien as the most expensive Ghanaian footballer of all-time following his move to Arsenal.
The Arsenal anchorman played an instrumental role as Ghana defeated Qatar in an international friendly on Monday, providing to assists.