The Greek defender, 25, has put pen to paper on a five-year deal after passing a medical early this week.
“I am really happy and excited to be here – and to be back in the Premier League with West Ham,” said Mavropanos.
“It’s an exciting time to be joining the club after last season’s Eruopa 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 will wear the No15 shirt at London Stadium on his return to English football after four years on Arsenal’s books earlier in his career.
The Gunners plucked the centre-back from PAS Giannina in 2018 but he rarely featured before spells with Nurnberg and Stuttgart, where he made a loan move permanent for £3m a year ago.
He joins Edson Alvarez and James Ward-Prowse in signing for West Ham during the summer transfer window.
“I’m pleased to welcome Konstantinos to West Ham United,” said manager David Moyes.
“We’ve been looking to add to our centre-half options this summer, so it’s good we’ve been able to bring in an international defender of his ability. We’re really looking forward to integrating him into the group.”