The former Aston Villa striker becomes Sam Allardyce's fifth summer signing as the Hammers eye an immediate return to the top flight.
Carew, who scored 52 goals in 133 games for Villa, told West Ham's official website: 'I am so pleased to be here.
'I am very happy to sign for such a big club. I am looking forward to being part of a good team here that will bring this great club back up to the Premier League again.'
The former Norway international has not been included in the West Ham squad who begin their Championship campaign against Cardiff later on Sunday.
Source: Daily Mail
Source: Daily Mail