Colima state spokeswoman Cecilia Ramirez says the legislature approved the constitutional change late Monday after seven of the state’s 10 municipalities approved the reform.
Ramirez says the law provides gay couples with numerous social benefits similar to those of married couples.
She says a survey found Colima residents opposed instituting same-sex marriage but did support giving gay couples some legal rights.
Currently, Mexican same-sex marriages are allowed in Mexico City, the southern state of Oaxaca and the state of Quintana Roo, home to the resort city of Cancun. The northern state of Coahuila began allowing same-sex civil unions in 2007.