A popular figure in his home country, Hernandez became Mexico's all-time leading goalscorer earlier this year after scoring 48 goals in 96 appearances, including at two World Cups. His two years at Bundesliga outfit Leverkusen also yielded 28 goals in 54 games.
Hernandez, known as 'Chicharito', arrives at West Ham following the high profile acquisitions of Argentine Pablo Zabaleta, England's Joe Hart and Austrian Marko Arnautovic.
The Mexican told the club's website that he could not wait to get started at the London Stadium.
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"For me, the Premier League is the best league in the world and when the opportunity came, I was desperate to sign for this club," he told the club's website.
"It was not a difficult decision. West Ham is an historic club, and very ambitious – this summer you can see with the players signed that they are looking to have a very good season."
Joint-chairman David Sullivan highlighted the international experience the former Real Madrid striker would bring to the club.
"He has a proven goalscoring record in the Premier League, La Liga and the Bundesliga, as well in the UEFA Champions League and at international level with Mexico," Sullivan said.
"We now have some fantastic options in attacking positions and, like all West Ham fans, I am looking forward to seeing Javier score goals in a claret and blue shirt next season."
($1 = 0.7675 pounds)
(Reporting by Christian Radnedge, editing by Pritha Sarkar)
