Facebook has introduced a new option allowing users to identify themselves as a gender other than male or female.
The basic user profile for members in the United States now includes a customisable category with a variety of gender identifiers, including "transgender," "intersex" and "fluid," the social networking company announced on Thursday.
"There's going to be a lot of people for whom this is going to mean nothing, but for the few it does impact, it means the world," Facebook software engineer Brielle Harrison, who is undergoing a gender transformation from male to female, told the Associated Press news agency.
Facebook has taken a milestone step to allow countless people to more honestly and accurately represent themselves.
Facebook said it wanted users to "feel comfortable being your true, authentic self."
The social networking site, which has more than 1.2bn members worldwide, said it worked with a leading group of gay and transgender advocacy organisations to develop the list of gender options, previously limited to male or female.
Rival site Google+ already had an "other" option for gender.
Custom genders are only available to those who use Facebook in US English, but the company plans to expand the range in the future.
Facebook's move will help support young people whose sexual identities do not conform to traditional societal norms, the Human Rights Campaign Foundation, a US-based gay advocacy group, told the Agence France-Presse news agency.
"Facebook has taken a milestone step to allow countless people to more honestly and accurately represent themselves," foundation president Chad Griffin said.
The Williams Institute, a think-tank based in Los Angeles, estimates that at least 700,000 individuals in the United States identify as transgender, an umbrella term that includes people who live as a gender different from the one assigned to them at birth.
Facebook's announcement generated some criticism from US religious group Focus on the Family, with representative Jeff Johnston telling the AP: "It's impossible to deny the biological reality that humanity is divided into two halves: male and female."