Darren Randolph has completed a £5million transfer from West Ham to Middlesbrough after the arrival of Joe Hart saw him fall down the pecking order at London Stadium.
The Republic of Ireland international made 22 Premier League appearances for the Hammers in 2016-17, helping Slaven Bilic's side finish 11th, but Hart's signing on loan from Manchester City left him unlikely to be first choice in the new season.
Randolph joined West Ham on a free transfer after being released by Birmingham City in 2015 and Bilic told the club's official website: "I would like to thank Darren for his service to the club over the past two years. He has been a fantastic professional and, as a player and a person, has the respect of everyone at West Ham.
"Darren is a very good goalkeeper, and I didn't want him to leave. However, I could not guarantee him regular first-team football and, with the possibility of playing at the World Cup for the Republic of Ireland next summer, he wanted to take up the offer from Middlesbrough.
"We didn't think it was right to deny him this opportunity to establish himself as a number one playing every week in the Championship, but I am sorry to see him go and I wish him all the very best in the future."
Randolph is Middlesbrough's sixth signing of the transfer window, following the club-record capture of Britt Assombalonga for £15m from Nottingham Forest.