The decision comes a day after a federal judge ruled that neighboring Kansas also was violating the U.S. Constitution by refusing to allow same-sex marriages. More than 30 U.S. states permit same-sex couples to marry.
"Marriage equality is now the law of the land in the state of Missouri," said Winston Calvert, the city attorney for St. Louis, who argued against the state's same-sex marriage ban. "This decision finally enforces that constitutional guarantee of equality for gay and lesbian couples."
In his ruling, St. Louis Circuit Judge Rex Burlison said that "the freedom to marry is a fundamental right and liberty deeply rooted in the history of the United States." He found that the state ban on same-sex marriage was not tied to a "legitimate government interest."
Marc Solomon, national campaign director for the Freedom to Marry advocacy group, applauded the ruling.
"As Missourians get to know married same-sex couples and their families, they will see clearly that their marriages are based on love, commitment and an interest in caring for their families," he said.
"Today's ruling adds to the powerful momentum of victories from a bipartisan array of federal and state judges as we work to secure the freedom to marry nationwide," Solomon said.
In the Kansas case, U.S. District Court Judge Daniel Crabtree granted a preliminary injunction Tuesday stopping Kansas from enforcing its ban on gay marriage and put the ruling on hold until Nov. 11 to giveKansas an opportunity to appeal.
Same-sex marriage has become legal in more than a dozen states since the U.S. Supreme Court said on Oct. 6 that it would not review recent U.S. appeals court decisions striking down state bans.
The number of states in which same-sex marriages may be performed jumped to 32 from 19 after the U.S. Supreme Court's announcement.
Reporting by Carey Gillam in Kansas City, Mo.