Hernandez only joined Sevilla in the summer from West Ham United but struggled for game time on his return to Spain, where he spent the 2014/2015 season on loan at Real Madrid.
The 31-year-old made 15 appearances in all competitions for Sevilla and scored three goals.
Hernandez will serve as a direct replacement for Zlatan Ibrahimovic after the former Sweden international left LA Galaxy for AC Milan at the end of his contract in December.
The former Manchester United striker will become the highest paid player in the MLS, earning around $6m a year.
"I'm arriving with the best club in MLS, so I'm very excited about this. I'm simply ready to finally put the shirt on," Hernandez said.
"I want to achieve a lot of things with the LA Galaxy because that's what prove and show. I signed with this club for [four seasons] so imagine four more stars on this badge. It'll be awesome."
Hernandez is Mexico’s all-time leading goalscorer with 52 goals from 102 caps.
LA Galaxy president Chris Klein said: "The LA Galaxy continue to pursue top talent across the world that will improve our team.
"Javier is one of most successful players in our region and an iconic figure in this community.
"The LA Galaxy have represented Los Angeles and have been the benchmark in Major League Soccer since 1996, and our pursuit of the world's best players for our club will continue to play a part in our success.
"We are excited to add Javier to our club and look forward to him representing the LA Galaxy in our stadium and in the Los Angeles community."
