The 23-year-old, nicknamed "Chicharito", made a big impact with United in his first season, scoring 20 goals from 45 appearances in all competitions after arriving in July 2010 from his hometown club Chivas de Guadalajara.
His agent Eduardo Hernandez told ESPN Deportes: "We have renegotiated Javier's contract and he is now committed for five years with Manchester United.
"The initial contract was good for Manchester and for Javier because it enabled them both to see if they could fit together and if he could achieve what was expected of him.
"Both sides have met their expectations and United were happy to reward Javier after his very good performances last season.
"I will not talk about numbers, but it is a five-year contract. It is a fixed salary with incentives based on the team's success in each of the different tournaments.
"Javier was not badly paid with his initial contract, but it was his first contract and, with respect, not comparable with the likes of Wayne Rooney and Nani. But today, I can say that the contract has just been signed and everyone is satisfied."
Hernandez becomes the 13th member of the United first team squad to sign a new deal in the last year.
His excellent form pushed Dimitar Berbatov out of a regular starting place and the Bulgarian striker's future at Old Trafford is still to be decided.
