The Argentine, 26, has put pen to paper on a deal until June 2023 and has the option to extend his stay at the London Stadium for a further two years.
He joined West Ham from Al Jazira Club in 2015 and has recently made his 100th Premier League appearance for the east London club.
Lanzini said: “I’m so happy to sign a new contract for West Ham. I think it’s the best decision for me.
“I’ve been here for a long time and I’ve said in an interview before that I like West Ham, I like the fans, I like the staff and when I came here all the people were comfortable with me. I like to stay here.
“It’s a big decision but also easy, because when you feel good in one club and you like the club and the club likes you, it’s easier to decide these things. For me, it was not difficult. It was easy, because I love it here.
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“I love London, I love the club, I am happy here, we have a very good team, we have a very good manager and the club wants to change and to be in more competitions in the future and that’s good for us.”
Lanzini began his career with River Plate but joined Brazilian giants Fluminese on loan in 2011 following River’s shock relegation from the Argentine top flight in 2011.
He returned to River for another two seasons before moving to the UAE IN 2014.
A year later, he joined the Hammers on loan where he wowed supporters and forged a good understanding with former Hammers midfielder Dimitri Payet.
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