Chile's socialist President Michelle Bachelet on Monday sent to congress a bill to legalize marriages and adoptions by same-sex couples.
It is the latest in a wave of liberal reforms that she was determined to launch before ending her term next year.
It follows last week's approval by the constitutional court of a new law easing the country's ban on abortion.
"It cannot be that old prejudices are stronger than love," she said as she publicly signed the bill on Monday to submit it for debate in the legislature.
"That is why, just as I promised, Chile is today taking a historic step by signing the marriage equality bill."
Chile legalized same-sex civil unions under Bachelet in 2015.
Bachelet's party controls congress, but the process of passing the law is expected to be drawn out and last into the next legislature after she leaves office in March 2018.
Her conservative rival Sebastian Pinera is favored to be elected as her successor in November elections.
He opposes the marriage reform.
"There should not be discrimination, but at the same time the essence of an institution such as marriage should be respected, which has always been about conserving the human race," he said.
Largely Catholic Chile is considered a relatively socially conservative country within Latin America.
It emerged only in 1990 from the right-wing dictatorship of Augusto Pinochet.
Divorce was legalized in Chile in 2004.