Arsenal have signed Japanese defender Takehiro Tomiyasu from Bologna, after right-back Hector Bellerin joined Spanish side Real Betis on loan.
The Gunners have also allowed winger Reiss Nelson to complete a season-long loan move to Dutch side Feyenoord.
Tomiyasu, 22, who can play at either centre-back or right-back, has signed a long-term contract with Arsenal.
He made 31 Serie A appearances last season but has featured just once for Bologna this campaign.
Calum Chambers and Cedric Soares have started at right-back in the north London club's three league games this season, all of which have ended in defeat.