"This way, equal marriage will be clear in our constitution," Pena Nieto said at an event marking the national day against homophobia.
Mexico City has authorized gay and lesbian marriages since 2009 and three of the nation's 31 states have followed suit. A fourth state, Campeche, has approved legislation but it has yet to be implemented.
The Supreme Court issued a landmark ruling last year declaring that it was unconstitutional for Mexican states to ban same-sex marriage.
Elsewhere in Latin America, Colombia became the fourth South American country to allow same-sex marriage when the constitutional court definitively legalized it last month.
Argentina was the first in South America to legalize same-sex marriage, in 2010, followed by Uruguay and Brazil three years later.