The ruling, which cited the U.S. Supreme Court's June decision to strike down a federal anti-gay marriage law, is expected to have sweeping implications for the broader restrictions of the law.
"The question presented is whether a state can do what the federal government cannot -- i.e., discriminate against same-sex couples ... simply because the majority of the voters don't like homosexuality (or at least didn't in 2004)," wrote Judge Timothy Black, referencing the measure passed in Ohio nearly 10 years ago. "Under the Constitution of the United States, the answer is no."
Monday's ruling came before the court after two married gay widowers sued the state to be recognized as married on the death certificates of their recently deceased spouses. Both couples were married in states that allow same-sex marriage, and argued Ohio's ban was discriminatory.
"Once you get married lawfully in one state, another state cannot summarily take your marriage away," Black wrote, alluding to the 1967 Supreme Court case Loving v. Virginia, which struck down anti-miscegenation laws and is generally regarded as the constitutional argument for marriage as a right.
The Ohio ruling is the third major court decision this week striking down state bans of same-sex marriage.
On Thursday, New Mexico's Supreme Court specified all state laws referencing marriage were defined to include married same-sex couples. And on Friday, a federal judge ruled Utah's ban on gay marriage unconstitutional, sending couples into a mad dash to wed before the law could be stayed awaiting appeal.
Celebrity Couples of 2014 [PHOTOS]