Earlier this month, the Premier League side met Hernandez's release clause and after holding talks with the Mexican international, they soon pushed through his transfer from Bayer Leverkusen.
Hernandez will make his Hammers bow in a friendly with Werder Bremen on Friday and speaking ahead of the game, the striker revealed why he decided to make the switch to East London.
The 29-year-old told Sky Sports News: "It was a little bit simple really. I'm in the best league in the world and West Ham showed me a lot of confidence and trust.
"West Ham is showing more ambition compared to the last seasons. They want to qualify for Europe, you can see that in the transfer window. That's why I'm here.
"They're one of the most historic teams in England, very ambitious this season and hopefully we can achieve our goals."
West Ham begin the new campaign away at Hernandez's former club Manchester United.