Rhode Island became the tenth state in the nation to legalize same-sex marriage after the state legislature approved the measure on Thursday. Gov. Lincoln Chafee is expected to sign the bill at 5:45 p.m. ET.
Last week both the Rhode Island House and Senate approved by large margins measures allowing same-sex couples to wed, but more votes were required after the legislation was tweaked. The law will go into effect August 1.
Iowa, New York, Vermont, New Hampshire, Massachusetts, Connecticut, Maine, Maryland, Washington and the District of Columbia allow same-sex marriage. Their combined populations, based on U.S. Census estimates for 2012, represent 15.8% of the U.S. population.
The addition of Rhode Island's 1,050,292 residents will nudge that up to 16.1%.
"Today's vote shows that marriage doesn't belong to a particular party or ideology, and increasingly, the public and politicians alike realize that only marriage provides loving couples and families with the protection they need and deserve," said Chad Griffin, the president of the gay rights group Human Rights Campaign.
"That the entire Rhode Island Senate GOP caucus joined with a vast majority of Democratic lawmakers to live up to the state's values in voting for marriage will inspire more momentum in more states," added Marc Solomon, the national campaign director for pro-same-sex marriage group Freedom to Marry. "It is time now for the Supreme Court to uphold these American values for all loving and committed couples."
Christopher Plante, the Rhode Island regional director for the National Organization for Marriage, opposed the law, saying it omitted important religious protections that have been included in other states' laws.
"Redefining marriage into a genderless institution to satisfy the demands of a small but politically powerful group is bad enough, but besides advocating a flawed policy HB 5015 and SB 38 contain a shocking lack of religious liberty protections," Plante wrote, saying the bill, at a minimum, should have included legal protections for individuals, businesses and groups who refuse to serve or participate in same-sex marriages.
In March, a divided United States Supreme Court heard arguments over the legality of two marriage laws - the federal Defense of Marriage act, which defines marriage as between one man and one woman, and California's Proposition 8, which bans same-sex marriage.
The nine justices hinted at disparate views on the hot-button issue, though it was far from clear how they will rule. A decision is expected in June.
CNN's Tom Watkins and Kevin Liptak contributed to this report.