Arsenal have signed Atletico Madrid's Thomas Partey before Monday's transfer deadline, the club announced.
The midfielder completed a medical on Monday after the Gunners triggered his £45 million release clause.
The Gunners had retained an interest in the 27-year-old throughout the summer but Atletico had refused to budge from their valuation with Arsenal struggling to finance the deal as they sought to trim a bloated squad.
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However, with Lucas Torreira finalising a loan move to Atletico in a separate transaction and Matteo Guendouzi joining Hertha Berlin on loan, the space was freed for Partey to complete a dramatic deadline day switch.
First-team manager Mikel Arteta made signing a central midfielder his top priority and had hoped to pull off a move for Lyon's Houssem Aouar but the Ligue 1 side were unwilling to lower their €60m asking price.
Arsenal had also enquired about Chelsea's Jorginho but the Blues did not want to strengthen a domestic rival. Arsenal had scouted Porto midfielder Danilo Pereira as well but opted not to follow up that interest and he moved to Paris Saint-Germain instead.
That chain of events meant Arsenal circled back to a decision on whether to trigger Partey's release clause.