The Mexico international will undertake the next challenge of his career in La Liga, two years after joining the Hammers from Bayern Leverkusen.
Javier Hernandez has completed a permanent €8 million (£7m/$9m) switch to Sevilla from West Ham on Monday.
The Mexico striker has signed a three-year contract at Ramon Sanchez-Pizjuan Stadium, bringing to an end his two-and-a-half season stay in east London.
The 31-year-old netted his final goal for West Ham during a 1-1 draw away at Brighton on August 17, which also marked his last appearance for the club.
West Ham have released an official statement on their website, which reads: "West Ham United can confirm that striker Chicharito has joined Spanish Primera Division club Sevilla on a permanent basis for an undisclosed fee.
"The club would like to thank Chicharito for his contribution and wish him all the best in his future career."
Hammers chairman David Gold also confirmed the news via his Twitter account, thanking Hernandez for his service before wishing him well in Spain.
He said: "Good morning. I can confirm that Javier Hernandez’s transfer to Spanish La Liga club Sevilla has been completed for an undisclosed fee. Good luck Javier"
Hernandez scored 17 goals in 63 games across all competitions for West Ham in total, helping the team secure a top ten spot last season after a 13th place finish in 2017-18.
The diminutive centre-forward has not quite been able to recapture his best form at the London Stadium, but he endeared himself to the fans with his work-rate and explosiveness in the final third.
Manuel Pellegrini decided not to include Hernandez in his squad for West Ham's 2-0 home win over Norwich on Saturday, ahead of his imminent return to La Liga.
The former Manchester United star left Old Trafford to join Real Madrid on loan for the 2014-15 season, before he secured a permanent switch to Bayer Leverkusen.
Hernandez had been linked with a move away from West Ham throughout the summer window, with a few incomings pushing him further down the squad pecking order.
The Hammers brought in Sebastien Haller from Eintracht Frankfurt, who has quickly become Pellegrini's first-choice striker, while Pablo Fornals and Albian Ajeti were also acquired from Villarreal and Basel respectively.
Hernandez joins a Sevilla side currently sitting third in the La Liga standings, with two wins and a draw from their first three matches of the season.