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 or adoption”.
Mr Acevedo, who had proposed the changes, said he had encountered opposition from conservative groups, including the Catholic Church.
The changes, which he called “a great step forward”, were approved by 19 votes to three.
They are expected to come into force as early as next week.
For the past seven years, same sex couples have been able to enter into civil unions in Coahuila, but have not been allowed to get married.
Mr Acevedo said the changes were crucial because “the way the law worked before, it infringed on the rights of homosexual people and now they will get their rights… everything which comes with marriage”.
Apart from Mexico City, same-sex marriages have also been taking place in the southern state of Quintana Roo, with couples taking advantage of the fact that the state’s civil code does not specify sex or gender requirements for marriage.