'Chicharito' becomes the Hammers' fourth signing of the summer.
Hernandez scored 39 goals in 76 appearances in two seasons at Bayer.
Spaniard played in the Bees' pre-season friendly win against Oxford United on Wednesday (19 July).
England goalkeeper will spend 2017-18 campaign in east London after sealing latest Manchester City exit.
However, French striker insists he will remain professional as speculation over his future continues to swirl.
Stoke City chairman Peter Coates says Austrian available "if the terms are right".
Iheanacho destined to leave City after being frozen out by Pep Guardiola.
Giroud, 30, has also been linked with Everton and West Ham.
Nigel Winterburn believes the London Stadium could offer both players a possible exit route.
The Frenchman has revealed his desire to win more trophies with the Gunners.
Bees club chief Rasmus Ankersen thinks it is too soon to tell whether the Spaniard will sign fresh terms.
Jimenez came close to signing for West Ham in the summer of 2015.
West Ham United and Lyon are also in the race to sign the Bayer Leverkusen striker.
Everton beat off competition from Arsenal and West Ham United to secure Nigerian prodigy.
Giroud is keen to ensure he receives regular game-time ahead of the 2018 World Cup.
"It's clear to most fans that we are desperate for centre-forwards," says Hammers co-chairman.
Everton, West Ham and Newcastle are also interested in signing the 23-year-old playmaker.
Bees chief Mark Devlin not surprised by interest in 25-year-old playmaker.
'I can bring Leeds United to where they belong': New boss Thomas Christiansen bullish over promotion
Christiansen believes his felxibility will be key to any success at Elland Road.
21-year-old is not thought to be part of Antonio Conte's first-team plans.
Hammers boss wants players who can make an instant impact following last's summer failed overhaul.
Olivier Giroud has been frustrated by his limited playing time with the Gunners.
Belgium international started just once in the league last season.
Player recruitment head Tony Henry concurs with Slaven Bilic that Hammers should target quality over quantity.
Spaniard is out of contract this summer but the club have an option to keep him until 2019.
Klopp was jubilant after Liverpool's 4-0 win over West Ham United.
Reds looking to edge closer to securing Champions League qualification.
Zabelata will leave the Etihad Stadium after nine years of service.
West Ham United vs Liverpool, Premier League 2016/17: Where to watch live, preview, betting odds and possible XI
West Ham have won their last three home games against Liverpool.
Liverpool drew 0-0 with Southampton at Anfield last weekend.