After scoring 28 goals in 83 games within 2 seasons for Derby, Wanchope was sold to West Ham United for £3.5 million on 16 July 1999.
Wanchope made his debut for West Ham on 28 July 1999, at Upton Park, against Heerenveen in the Intertoto Cup. His first West Ham goal came on 4 August 1999 in the away leg of the same tie. He scored 15 goals in 47 appearances in all competitions; his final game coming on 14 May 2000, at home to Leeds United. During his one year at Upton Park Wanchope formed a deadly partnership with Paolo Di Canio, the two scored a combined 31 league goals in the 1999–2000 season, and West Ham finished 9th in the Premier League, putting them in the top nine for the third consecutive season. He was eventually sold to Manchester City at the start of the 2000–01 season, following the arrivals of Davor Šuker and Frédéric Kanouté, for a fee of £3.65million.