The measure also gives people the right to specify how their gender is listed at the civil registry when their physical characteristics don't match how they see themselves.
Senators approved the Gender Identity law by a vote of 55-0, with one abstention and more than a dozen senators declaring themselves absent — the same margin that approved a "death with dignity" law earlier in the day.
President Cristina Fernandez threw her support behind the law and is expected to sign it. She has often said how proud she is that Argentina became Latin America's first nation to legalize gay marriage two years ago, enabling thousands of same-sex couples to wed and enjoy the same legal rights as married heterosexual couples.
For many, gender rights were the next step.
Any adult will now be able to officially change his or her gender, image and birth name without having to get approval from doctors or judges — and without having to undergo physical changes beforehand, as many U.S. jurisdictions require.
"It's saying you can change your gender legally without having to change your body at all. That's unheard of," said Katrina Karkazis, a Stanford University medical anthropologist and bioethicst who wrote a book, "Fixing Sex," about the medical and legal treatment of people whose physical characteristics don't fully match their gender identity.
"There's a whole set of medical criteria that people have to meet to change their gender in the U.S., and meanwhile this gives the individual an extraordinary amount of authority for how they want to live. It's really incredible," she said.
When Argentines want to change their bodies, health care companies will have to provide them with surgery or hormone therapy on demand. Such treatments will be included in the "Obligatory Medical Plan," which means both private and public providers will not be able to charge extra for the services.
"This law is going to enable many of us to have light, to come out of the darkness, to appear," said Sen. Osvaldo Lopez of Tierra del Fuego, the only openly gay national lawmaker in Argentina.
"There are many people in our country who also deserve the power to exist," Lopez said.
Children also get a voice under the law: Youths under 18 who want to change their genders gain the right to do so with the approval of their legal guardians. But if parents or guardians want a gender identity change and don't have the child's consent, then a judge must intervene to ensure the child's rights are protected.
Argentina need not worry about vast numbers of people demanding sex changes, Karkazis predicted.
"This isn't going to create a huge demand on the national health system for these procedures. They're difficult, painful, irreversible. And this is why many people don't do it," she said.
But because the law says people can legally change their identities without having to undergo genital surgery or hormone therapy, these changes can be more benign and even reversible, if some day the person's self-image changes.
Other countries, including neighboring Uruguay, have passed gender rights laws, but Argentina's "is in the forefront of the world" because of these benefits it guarantees, said Cesar Cigliutti, president of the Homosexual Community of Argentina.
"This is truly a human right: the right to happiness," Sen. Miguel Pichetto said during the debate.
I'm not well-versed on trans rights around the world, but this sounds like it puts Argentina right at the top now. (Certainly ahead of the US.)