McDaniel, a Democrat serving his final year as the state's top attorney, became the first statewide official in Arkansas to back same-sex marriage.
"I want to tell you I do support marriage equality and I do believe Arkansans should have the right to be equal in the eyes of the law," said McDaniel, speaking at the Associated Press Managing Editors convention.
Voters overwhelmingly approved a constitutional amendment defining marriage as between a man and a woman, but that ban and others nationwide are facing legal challenges.
Seventeen states allow gay marriage, and federal judges have struck down bans in Michigan, Utah, Texas, Oklahoma and Virginia. An Arkansas judge is expected to rule by Friday in a lawsuit challenging Arkansas' ban.
"I'm going to zealously defend our constitution, but at the same time I think it's important to let people where I stand on the matter," McDaniel told the AP after his speech.
McDaniel said during a question and answer session with editors that there wasn't any single incident that changed his mind about gay marriage.
"It's become more and more difficult for me to accept the idea of anyone being treated as a second class citizen," McDaniel said.
McDaniel announced his support for gay marriage after criticizing U.S. Attorney General Eric Holder for telling his state counterparts in February they weren't obligated to defend laws in their states banning same-sex marriage if the laws discriminate in a way forbidden by the Constitution.
