West Ham signed Greece defender Konstantinos Mavropanos from Stuttgart in a deal worth a reported £17 million ($21 million) on Tuesday.
Mavropanos returns to the Premier League after making seven top-flight appearances during a three-year spell at Arsenal.
The 25-year-old spent two seasons on loan in the Bundesliga with Stuttgart before making the move permanent last year.
"I am really happy and excited to be here - and to be back in the Premier League with West Ham United," Mavropanos said after agreeing a five-year contract with the east London club.
"It's an exciting time to be joining the club after last season's Conference League win - and I am really looking forward to the next step in my career. I can promise the fans I will give everything for them and the shirt."
Mavropanos, who has 19 caps for Greece, will compete with Kurt Zouma, Nayef Aguerd, Angelo Ogbonna and Thilo Kehrer for a place in West Ham's defence.
He was signed after a move for Manchester United centre-back Harry Maguire broke down.
Mavropanos is the third major arrival at the London Stadium since the end of last season following the additions of Edson Alvarez and James Ward-Prowse.
There had been suggestions last week that Mavropanos' move had broken down after an issue with his medical but he has now made the switch.
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