Along with male and female, passengers flying United Airlines can now identify themselves as Mx.
The move makes United the first US carrier to offer non-binary gender options throughout the booking process.
Those who identify as non-binary can now check the honorific Mx box rather than Mr or Ms.
Also added to United customer profile choices are U for undisclosed or X for unspecified, alongside M (male) and F (female), to correspond with a passenger's passport or identification papers.
The new booking options were created in partnership with Human Rights Campaign and The Trevor Project, and the campaign included employee training.
"At the Human Rights Campaign, we believe being acknowledged as the gender you identify with is part of treating everyone with dignity and respect," said Beck Bailey, acting director of the Workplace Equality Program in a statement.
"By providing non-binary gender selection for ticketing and the gender-inclusive honorific 'Mx' in user profiles, United Airlines is taking an important step forward for non-binary inclusion."
American Airlines and Delta Air Lines are also expected to follow suit.
Last month, Airlines for America and the International Air Transport Association (IATA) approved a new international standard for non-binary passengers that will be effective June 1.
