Arsenal have sold defender Krystian Bielik to Derby for a projected £10m.
The Gunners will receive an initial fee worth £7.5m from Derby for the 21-year-old.
Bielik spent last season on loan with Charlton, helping them secure promotion to the Championship via the play-off route.
He joined Arsenal from Legia Warsaw in 2015 for £2m, but has only made two senior appearances for the club since then.
Bielik said: "I'm glad I joined Derby. You have a great coach here. I'm looking forward to it a lot because I know I can learn a lot from the gaffer.
"I want to learn a lot from the manager because I think he can help me to improve myself.
"I can't wait to join the lads on the pitch and I can't wait for the first game."
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