Defender George McCartney has signed a two- year contract to secure his return to West Ham for a third time.
The Northern Ireland man has spent time with West Ham from 2006-08 as well as a brief stint with the Hammers last season on a short-term contract during which he helped the club gain promotion to the top flight.
"I am delighted to be coming back to West Ham and this time on a permanent basis," McCartney said. "It was great to be part of the side that won promotion for the club last season.
"It was a fantastic way to end the campaign last year and I am really looking forward to being back in the Premier League with West Ham."
The 31-year-old becomes the fourth signing of the summer for manager Sam Allardyce, joining Jussi Jaaskelainen, Stephen Henderson and Mohamed Diame.