The 27-year-old is going would have cost 50 million euros.
Partey is said to earn £ 250,000 a week at the Emirates, where he is expected to solidify a midfielder that sometimes lacks bite and creativity this season.
Manager Mikel Arteta has been looking all summer for someone who could strengthen this area of the pitch. Once it was clear Lyonnais would not lower the asking price for the coveted Frenchman Houssem Aouar, all eyes were on Partey, who has been linked to the North London lot for several years.
“We have been watching Thomas for some time, so we are delighted to add such a quality player to our squad,” said Arteta. “He is a dynamic midfielder with great energy. He brings a lot of experience from a top-level club which has played at the highest level in La Liga and the Champions League for several years.”
Partey is leaving Atletico after making 188 appearances for the club in eight years.
He will join his new teammates next week after returning from the international break with Ghana.