On-loan defender Craig Dawson will join West Ham on a permanent transfer from Watford at the end of the season.
The 30-year-old has made 15 Premier League appearances, triggering a clause in the initial loan deal for the Hammers to make the transfer permanent.
Since the centre-back's debut in December, West Ham have lost just three league games and currently sit fourth in the table.
The ex-West Brom defender's new deal runs until the summer of 2023.
"Craig's performances, attitude and the impact he's had demonstrate why he deserves a permanent deal," manager David Moyes said.
"He arrived with a wealth of Premier League experience and that's been really important to us.
"He had to be patient but he's comes in and grabbed his opportunity with both hands. We are always looking to improve the squad and Craig joining us permanently certainly does that."
Find all the latest football transfers on our dedicated page.