Arsenal have signed Thomas Partey from Atletico Madrid on Deadline Day after meeting his £45m release clause.
Partey signed a long-term deal at the Emirates after he completed his medical and paperwork just before Monday's 11pm deadline.
He will wear the No 18 shirt and will join up with his new team-mates next week after he returns from international duty with Ghana.
Partey has been a key player in Diego Simeone's midfield since breaking into the first team from Atletico's academy in 2015.
Mikel Arteta said: "We have been watching Thomas for a while, so we're now delighted to add such a high quality player to our squad.
"He is a dynamic midfielder with great energy. He brings a lot of experience from a top club that has competed at the highest level in La Liga and the Champions League for several years.
"We're very impressed with his attitude and his approach to the game. He's an intelligent footballer and we're looking forward to him integrating into our system and contributing to the progress we're building at the moment at the club."
"Dear Atletico supporters, today I only have words of gratitude for you," Partey said in a farewell message on Instagram on Tuesday. "Atletico has been my home for a few years now and this family will always be a part of me.
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Querida afición rojiblanca, hoy solo tengo palabras de agradecimiento con vosotros. Desde hace ya varios años, el Atlético de Madrid ha sido mi casa y una parte de esta familia siempre estará conmigo. Quiero dar las gracias al club por la confianza depositada en mi desde el primer día , pero sobre todo, quiero daros las gracias a vosotros, a esta maravillosa afición que me acepto desde el primer momento y que ha demostrado que está tanto en las buenas como en las malas y que verdaderamente nunca deja de creer. El atlético de Madrid no es solo un club, es una familia. Gracias ♥️
"I want to thank the club for the trust they gave me from the first day, but above all, I want to thank you, this wonderful fanbase that accepted me from the first moment and demonstrated me that, both in the good times and the bad times, it really never stops believing.
"Atletico Madrid is not just a club, it's a family. Thank you."
The Gunners also agreed permanent deals for defenders Cedric Soares and Pablo Mari, who had been on loan at the club, extended the loan deal for Dani Ceballos from Real Madrid for a second year, and signed goalkeeper Alex Runarsson from Ligue 1 club Dijon on a four-year deal.
In recent weeks, Arsenal also moved for Lyon midfielder Houssem Aouar but after their offer was rejected they turned their attention back to the Atletico midfielder.
Lucas Torreira has moved in the opposite direction to Atletico Madrid in a loan deal until the end of the season and Matteo Guendouzi has signed for a German club Hertha Berlin on a season-long loan.
Analysis: Partey strengthens Arsenal
Sky Sports' Paul Merson:
"I think Thomas Partey is a top drawer player. I nearly fell off my chair with £45m, I don't think that's a lot of money at all for the quality of player.
"One thing he brings as well is discipline. Atletico Madrid are probably one of the most disciplined teams in Europe and he will do that. He's not a midfielder that goes running all over the place, he's very disciplined and plays in a disciplined team and I think he will be a great signing.
"I think what Mikel Arteta is doing, he's trying to get rid of the players who aren't involved, he doesn't want them hanging around and he wants them out. Fair play, I like what Arsenal are doing."
At Atletico, his ability was apparent long before he was sent on loan. In fact, Javi Baños, a former youth coach there, only needed to see him play once, in a friendly for their U18s against Getafe, to be convinced that his trial period should be made permanent.
"You could quickly see that he was a player with attributes which were very advanced for his age," Baños tells Sky Sports. "Above all, it was the precision of his passing. He was very effective in terms of his delivery. He also covered a lot of ground and positioned himself well on the pitch tactically."
'A surprise Deadline Day' at the Emirates
Sky Sports News reporter Dharmesh Sheth:
"It was a bit of a surprise with Arsenal. We didn't think at the beginning of the day they would get the midfielder they wanted, there was a lot of talk about Houssem Aouar in the last 48 hours, and then the one they were interested in from the very beginning, Thomas Partey, they knew they had to meet that £45m release clause, got triggered.
"They didn't have to deal with Atletico Madrid at all because how the release clause works is you go to La Liga's headquarters and put the money down. I'm not sure it was cash, I think it may have been just a cheque! Then you can talk to the player. They've got it done in quick time because it was a race against time for Arsenal to try to get this one done.
"It doesn't look like there was a deal sheet required for this, we've been told he will wear the No 18 shirt, Thomas Partey, he's joined on a long-term contract."
Arsenal midfielder Matteo Guendouzi has joined Hertha Berlin on a season-long loan.
The French midfielder has been surplus to requirements at Arsenal since their 2-1 defeat to Brighton on June 20, where he was involved in a post-match altercation with Neal Maupay.
Guendouzi, who joined the Gunners from Lorient in 2018, is under contract at the Emirates Stadium until 2022.
The 21-year-old has played 82 times for the club in all competitions, scoring once and making five assists since his debut in August 2018.