Supporters predicted the Democratic governor's full backing could give the measure the push needed for passage. A same-sex marriage bill cleared the state Senate this year, but was pulled from the floor of the House of Delegates after vote-counters determined they were a few delegates shy of a majority.
"I don't think that is an injustice that should be allowed to stand," O'Malley said. If the effort succeeds, Maryland would become the seventh state to allow same-sex nuptials. The District of Columbia also allows such marriages.
O'Malley's announcement came on the same day that President Barack Obama certified the repeal of Don't Ask Don't Tell, the policy that forced gay members of the military to hide their sexual orientation or face expulsion.
And it came two days before New York is scheduled to issue its first marriage licenses to same-sex couples. New York Gov. Andrew M. Cuomo, a Democrat elected last fall, led the effort there; his success has sparked speculation that he could be a presidential contender in 2016.
O'Malley and others advocates said they would try to learn from the Empire State.
"It is a fundamental truth that with every accomplishment further accomplishments appear possible," O'Malley said. A lifelong Catholic, he spoke admiringly of the protections for religious groups written into the New York law, which were similar to those in the Senate passed version of Maryland's bill.
He did not describe how his bill might differ from the one that was debated this year in Maryland.
The Maryland Catholic Conference called O'Malley's the decision "regrettable" and said the "the moral and social impacts of redefining marriage would be pervasive and severe."
Senate Republican Leader Nancy Jacobs vowed to "fight vehemently" against the governor's bill. She said she would support extending "legal rights" to gay couples, but would stop at recognizing same-sex marriage. She said O'Malley's bill "threatens the institution of the traditional family."
Rumors that O'Malley would sponsor a same-sex marriage bill next year had circulated for weeks. The announcement completes a 180-degree turn for a politician who said just five years ago that he was "raised to believe that marriage is between a man and a woman."
He said at that time that he opposed gay weddings but favored civil unions for same-sex couples — an arrangement that offers some tax benefits and legal protections.
O'Malley was asked three times during the news conference Friday to explain his personal evolution on the issue. He said that he did not want to spend too much time "rolling around" in his individual story, but said that his upbringing in the Catholic Church and attending Catholic schools had be influential in his early thinking.
Others played down the significance of O'Malley's shift. Del. Maggie McIntosh, a powerful Baltimore delegate who was the first openly gay member of the General Assembly — called O'Malley a "steadfast partner."
She pointed to O'Malley's record as mayor of Baltimore, where he implemented a law that prohibited employers and landlords from discriminating against employees and tenants based on sexual orientation, and in Annapolis, where he has signed bills that guarantee the right of same-sex partners to visit each other in the hospital and extend some tax rights.
O'Malley's change of heart on same-sex marriage tracks closely with public opinion. A Sun poll in 2008 showed that 19 percent of likely voters supported the arrangement (39 percent said they would back civil unions.) A Washington Post poll in May 2010 found that support had risen to 46 percent.
"The ability to come together around marital equality is one of the faster moving issues that we've seen in our country for some time," O'Malley said Friday.
Though both houses of the General Assembly are dominated by O'Malley's fellow Democrats, inclusion on the governor's legislative agenda is by no means a guarantor of success.