Thilo Kehrer is a defender from Paris St-Germain.
He has started for his country 22 times in the past year.
The Hammers have signed six players in the summer, including goalkeeper Alphonse Areola.
"I'm very excited to sign for West Ham United," said Kehrer, who has joined for a reported £10.1m. "This is the best league in the world and I'm excited about coming to play in the Premier League.
The manager told me how he thinks I will fit in at the club. I want to integrate myself into the group and enjoy playing for West Ham.
He played 34 times for the French champion last season and has the option of a further two years on his contract.
He won three Ligue 1 titles and two French Cups.
"He's a talented player, who can play in a number of defensive positions, adding strength and depth to our current options," said the Hammers boss.
West Ham are in talks with a club in England.
Ben Johnson and Kurt Zouma have been used in central defence this season due to injuries to Craig and Angelo.
There's more to come.
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