"I am very happy to join West Ham United", Chicharito said in the release.
Hernandez, 29, Mexico's all-time leading scorer, has signed a three-year deal to become the Hammers' fourth new arrival of the summer.
"Everyone knows that West Ham is a big club, with big history, and I'm happy to be a part of it now", Arnautovic told the club website after leaving Stoke, where he played for four seasons, scoring 22 Premier League goals.
Marko Arnautovic had an average shot accuracy of 44% in the Premier League last season
"It was not a hard decision".
"You can see in the summer transfer window that they spend very well, with good players because we want to achieve a lot of stuff and be better than last season".
Now, Bilic and the West Ham board look set to regain some of that swagger by bringing in a mixture of talent and experience to the club.
Bought from Inter past year for more than £7m, Genoa's Diego Laxalt appears to be once again attracting interest, this time from the Premier League.
The deal for Hernandez follows the club record signing of Marko Arnautovic, who signed a five-year deal for a fee of £20m, which could rise to £25m.