New York City residents can now change the gender on their birth certificates to 'X'.
The new bill means the city is the only place in the state to offer a third non-binary option on birth certificates.
People are able to make the alteration without a note from a medical professional. It also allows parents to assign newborns with the gender-neutral 'X' marker.
The law - which was passed earlier in the year - came into effect on New Year's Day and follows in the footsteps of similar bills permitting a third gender option in California, Oregon and Washington state. New Jersey will also introduce the 'X' option next month.
Speaking after the bill was passed, the mayor of New York, Bill de Blasio, said the city was 'a place where everyone can belong'.
He said: "Should everyone be able to tell their government who they are and not the other way around? That's why this is so important.
"One thing that New York City is really good at is being a place for everyone. One of the things we are renowned for all over the world is [that] it's a place where we can all belong.
"So many folks for so many years had to deal with documents that did not reflect their reality. I can only begin to imagine what that challenge was like, what that pain was like. That's why Intro 954 is so important."
To all trans and non-binary New Yorkers: We see you, hear you and respect you. Starting in 2019, all New Yorkers will be able to change their gender on their birth certificate to M, F or X - without a doctor's note. pic.twitter.com/we4TbCVdhJ
- NYC Mayor's Office (@NYCMayorsOffice) December 29, 2018
He continued: "In this city, which has been the wellspring of the movement for equality for so long, the home of Stonewall and the modern LGBTQ movement, it is fitting that we lead the way here again.
"To anyone who feels they have been misunderstood or ignored or mistreated, because of their choice of their own identity, a simple message - you be you. Live your truth, and know that New York City will have your back."
In 2014, New York City abolished a law that only allowed people to alter the gender on their birth certificate if they underwent reassignment surgery. In lieu of this, the most populous city in the US required only an attestation from a medical practitioner.
The new law means no such medical documents are required now either.