Mexico City Mayor Claudia Sheinbaum has announced that pupils attending state schools will no longer have to wear gender-specific uniforms.
"Boys can wear skirts if they want and girls can wear trousers if they want," Mayor Sheinbaum said.
She added that the measure would create "a condition of equality, of equity".
Previous guidelines stated that "just as the skirt is the basic garment of a girl's daily school uniform, so trousers are for boys".
Mayor Sheinbaum said that that kind of thinking "had passed into history" and that the new measure would come into effect "immediately".
Una ciudad de derechos lleva la igualdad a todas las esferas. Con el secretario de la @SEP_mx @emoctezumab anunciamos el uniforme escolar neutro en la Cd. La falda no será exclusiva para las niñas y el pantalón para los niños. Acciones sencillas para promover igualdad de derechos pic.twitter.com/SlohIh5T5P— Claudia Sheinbaum (@Claudiashein) June 3, 2019
Ms Sheinbaum, from the left-wing Morena party, made the announcement while visiting a primary school in the city centre.
It was welcomed by transgender activists who said it would help children who until now had to use a uniform they may not have identified with.
Education Minister Esteban Moctezuma backed the mayor's move and said he was sure many other states would follow suit.
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It is not the first time Mexico City is ahead of the rest of the country in introducing more liberal norms.
In 2010, it became the first area in Mexico to issue marriage licences to same-sex couples. Since then, more than a dozen other states have legalised it.
Last month, President Andrés Manuel López Obrador said he would push for same-sex marriage to be made legal across the country.
Despite efforts by women activists to push for more equality, discrimination and violence against women remain a huge problem in Mexico.