The Hammers have agreed to sign the Ecuador international from Pachuca following his fine displays at the World Cup, and he will become the club's fifth signing of the summer
The 25-year-old has signed a five-year deal at Upton Park and will become Sam Allardyce's fifth signing of the summer following the arrivals of Aaron Cresswell, Cheikhou Kouyate, Mauro Zarate and Diego Poyet.
Valencia shone at the World Cup with Ecuador, scoring all three of his country's goals, and also got himself on the scoresheet against England when the two sides met in Miami prior to the tournament.
The 24-year-old bagged 23 goals in all competitions last season and will likely form a strike partnership with Andy Carroll at Upton Park this season.
The Hammers' interest had been ongoing as they looked to improve on a Premier League goal tally of just 40 in 38 matches in 2013-14, and the arrival of Teddy Sheringham as a coach has hinted at a more attacking style of play from Allardyce's side.
Though the fee for the forward has been reported as undisclosed, Pachuca chairman Jesus Martinez has previously said that Valencia would only be sold for the highest transfer fee in Mexican football history.
The Irons are believed to have paid in the region of €19 million for the South American, and his competitive debut could come against Tottenham on August 16.