Sevilla will pay West Ham a transfer fee of €7.8m (£7m) for the 31-year-old Mexican plus an additional potential €1m (£900,000) in add-ons.
The Hammers will pay the normal 5% of the transfer fee to FIFA in a solidarity payment leaving a net transfer payment of around £6.6m.
Hernandez has taken a large pay cut as he was desperate to return to a Spanish speaking country having turned down lucrative offers from Russia & Turkey.
With ten months remaining on his contract, the Hammers will save another £6.4m in wages as Hernandez was the highest earner in the first-team squad.
Chicharito will lose all claim to his remaining signing-on/loyalty bonus after slapping in a transfer request on Thursday.
West Ham signed striker Chicharito from German club Bayer Leverkusen for a £16 million fee in July 2017 on a three-year contract reported to be worth up to £145,000 per week to the 31-year-old Mexican.