Midfielder Thomas Partey has joined Arsenal from Atletico Madrid.
The Gunners have been in the market for a midfielder throughout the window, and have been heavily linked with a move for Partey and Lyon's Houssem Aouar.
The club met Partey's £45m release clause on deadline day which paved the way for the deal to go through, with Atletico confirming on their official Twitter account that his contract had subsequently been terminated as a result.
"We have been watching Thomas for a while, so we’re now delighted to add such a high quality player to our squad," Mikel Arteta told Arsenal's official club website.
"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.”
The 27-year-old joined Atletico in 2011 and broke into the first team in 2015. He made 35 La Liga appearances for Diego Simeone's side last season, scoring three goals.
Partey becomes Arsenal's sixth permanent signing of the window following the arrivals of Willian, Pablo Mari, Alex Rúnarsson, Gabriel and Cédric Soares.
“Thomas has all the right attributes to be a top player for Arsenal," added Arsenal technical director Edu. "He is a leader on and off the pitch and he is exactly the kind of player and person we want at the club. We already feel like we know Thomas very well based on the work we have done analysing his performances closely in recent times.
"With his all-round game and positive aggressive style, he will be a brilliant addition to our squad. We’ve made a strong start to the season and we want to continue to build on this with Thomas now part of our club.”
Midfielder Lucas Torreira has gone in the opposite direction, joining Atletico on a season-long loan.