The 29-year-old Portuguese international has signed a 3-and-a-half-year contract after passing a medical last night and will go into the Hammers squad for the FA Cup game against Brighton on Saturday.
Boa Morte started his career with Sporting Lisbon before moving to Arsenal in 1997. He spent two years at Highbury under Arsene Wenger but found first team opportunities limited. He then had a short spell at Southampton before moving to Fulham in 2000.
Boa Morte is a powerful left-sided winger and looks set to be Alan Curbishley's ideal replacement for the under-performing Matthew Etherington.