Chaffee, an independent, signed the bills Thursday after the House gave final approval, The Providence Journal reported.
The bills allow same-sex weddings beginning Aug. 1 and allow couples in civil unions to change their status to married.
"Rhode Island is one of the highest Roman Catholic states in the country if not the highest," Chaffee said on MSNBC Thursday. "And still, it was 56-15 in the House and 26-12 in the Senate."
He said he thought the measures passed because public opinion was changing, especially among young people.
"Young people [are] very, very tolerant, accepting the diversity that comes with gay marriage and actually welcoming of it," he said. "That's a big political dynamic."
Rhode Island joins Connecticut, Iowa, Massachusetts, Maryland, Maine, New Hampshire, New York, Vermont, Washington and the District of Columbia in sanctioning same-sex marriage.