Summerville, the Championship player of the year last season, becomes English Premier League club West Ham's fourth summer signing after agreeing a five-year deal.
The 22-year-old, whose fee could reportedly rise to $67 million with add-ons, was the third-highest scorer in the second tier last season with 19 goals, and also made nine assists.
Summerville scored 25 goals in 89 appearances in total for Leeds, who lost in last season's Championship play-off final, after joining from Feyenoord in September 2020.
The Londoners, who appointed former Real Madrid and Spain boss Julen Lopetegui as head coach in May following David Moyes' departure, have also been linked with Germany striker Niclas Fullkrug.
Fulham have completed the signing of defender Jorge Cuenca from Villarreal for an undisclosed fee.
The Spanish centre-back, who has signed a four-year deal, has represented Alcorcon, Barcelona, Almeria, Getafe and Villarreal since his professional debut in 2017.
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