The defensive midfielder is delighted to sign after his release clause was triggered hours before the transfer deadline closed.
He is now a Gunner, and he sent a social media message through the Arsenal channel to his new supporters.
"Hello, Gunners, thanks so much for the love, for the messages I've received," Partey said in a message on Arsenal's Twitter account.
"I can't wait to be part of you guys, I can't wait to join team.
"It's a great feeling to be part of a family like this.
"Thanks so much to everyone for the love, I feel so grateful and so amazed to be part of you.
"I can't wait to join you guys and don't forget, no Thomas, no Partey."