New York City residents who do not identify as male or female can now opt for a third gender on their birth certificates.
Under new rules that came into effect on New Year's Day, New Yorkers can choose 'X' instead of the traditional male or female.
The new law was approved by the New York City Council and Board of Health last September.
The revision also makes it easier for transgender people to change their gender as it removes the requirement for a doctor's note or health care provider's affidavit.
New York City's Democrat mayor Bill de Blasio said at the time that the change would 'allow transgender and gender nonconforming New Yorkers to live with the dignity and respect they deserve'.
Residents of New Jersey, California, Washington, and Oregon can already choose 'X' on their birth certificates, while Washington DC permits the designation on driver's licenses.
The change came into effect on the same day as Germany introduced a new 'diverse' option for intersex people.
The new law, passed last month, requires that people who want to identify as 'diverse' on official documents provide a doctor's certificate.
Until now, people who did not identify as male or female had to leave the gender category blank.
The law was changed after Germany's Federal Constitutional Court told the government it must either introduce a third gender category option or drop gender entries from official documents altogether.
Intersex people are born with sexual characteristics that are not typically male or female. According to the UN, up to 1.7 per cent of the world's population may be intersex.