Acting under legislation passed in April, Rodrigo Borda and Sergio Miranda, aged 39 and 45, flashed big smiles as they addressed a gaggle of reporters upon leaving a civil registry in Montevideo.
"We are celebrating it and sharing it because this law establishes that we all have rights. There are no first and second class citizens," said Miranda.
The men said they will pick a date for their actual wedding later this month.
Argentina legalized gay marriage in 2010, becoming the first Latin American country to do so.
After Uruguay did so with broad support from all its political parties, Brazil gave the green light to same sex marriages in May.
They have also been legal in Mexico City since 2009, although not in the rest of the country.
Voice of Rusia, AFP