Arsenal burned off five rivals to sign Spezia defender Jakob Kiwior.
The Poland international, 22, passed his medical on Saturday and is due to complete his transfer today.
Arsenal will pay 25m up front plus a further 5m in bonuses.
TMW says Arsenal outbid Borussia Dortmund for Kiwior, with the Germans having a 22m offer rejected last week.
Leicester City and Brentford also made attempts, along with RB Leipzig and Atletico Madrid.