In January 1998, Sinclair moved to West Ham United for a combined fee of £2.3 million; £2.7 million minus West Ham players Iain Dowie and Keith Rowland moving to Q.P.R as part of the deal. He scored seven goals in the remaining 14 games to help West Ham achieve an eighth-place finish in the Premier League. He was a regular over the next two seasons, but a serious knee injury ruled him out of the final four months of the 2000–01 season. His absence coincided with a loss of form for the team, and by the end of the season manager Harry Redknapp had been sacked.