The 25-year-old has signed a five-year deal at Upton Park and will become Sam Allardyce's fifth acquisition of the summer following the arrivals of Aaron Cresswell, Cheikhou Kouyate, Mauro Zarate and Diego Poyet.
Valencia shone at the World Cup with Ecuador, notching all three of his country's goals, and also got himself on the score sheet against England when the two sides met in Miami prior to the tournament.
WORLD CUP: Beautiful people | Germany homecoming
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.
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 soccer history.
The Irons are believed to have paid in the region of 15 million pounds for the striker, and his competitive debut could come against Tottenham on Aug. 16.