In a 9-2 vote last week, Mexico's Supreme Court upheld the portion of Mexico City's same-sex marriage law that lets married gay and lesbian couples adopt.
In two other rulings, the court had upheld the main part of the marriage law and ruled that same-sex couples who marry in Mexico City are validly married everywhere in the nation, in all 31 states.
Human Rights Watch said the trio of rulings confirmed "that the state cannot withhold any legal rights on the grounds of a person's sexual orientation and gender identity."
"This decision will have resonance for courts throughout the continent for protecting the basic human rights of LGBT people," said the group's Juliana Cano Nieto.
Mexico City's legalisation of same-sex marriage and adoption had been targeted by the federal attorney general, whose office said the moves undermined "family" and the interests of children.
The court decided, however, that married heterosexuals are just one kind of "family" and that children's interests are served by having a loving family regardless of their parents' sex.