Coahuila's congress approved changes to the civil code which give same-sex couples the same rights as heterosexual couples, including adoption.
In Mexico, laws on same-sex unions fall under state legislation, and a number of them have divergent rules.
In 2009, Mexico City became the first entity to legalise same-sex marriage, while the state of Yucatan banned it.
Leftist Congressman Samuel Acevedo said lawmakers had changed the civil code which defined marriage as a "union between a man and a woman for the purpose of procreation" to "a union between two people with the possibility of procreation . . .