Bellamy, who turns 28 on Friday, has moved to Upton Park after it was made clear he had no future at Anfield following the arrival of Spain international Fernando Torres.
The transfer was agreed at the end of last week but was delayed over the weekend as Bellamy thrashed out contractual issues with Liverpool.
The Wales captain is West Ham's fourth summer signing and takes manager Alan Curbishley's spending since he arrived at Upton Park in December to nearly £40million.
The deal for Bellamy breaks West Ham's previous transfer record of £7.25million, which the Hammers paid Norwich for Dean Ashton in January 2006.
Bellamy joins midfielders Scott Parker and Julien Faubert plus goalkeeper Richard Wright in signing for the Hammers this summer.
Curbishley had previously targeted Darren Bent and had a £17million offer accepted by Charlton, only for the England striker to reject the Hammers for Tottenham.