In what supporters have hailed as an historic moment in civil rights history, Mr Obama reversed his position on the controversial social issue just six months before the November election.
"I've just concluded, for me, personally, it is important for me to go ahead and affirm that I think same-sex couples should be able to get married," Mr Obama said in an interview with ABC News America.
However, Ms Gillard says her views have not changed and she remains opposed to gay marriage in Australia.
"I've made my mind up and my position on this is well known," she said.
Audio: Obama backs gay marriage (AM)
"It reinforces this is a matter that people form their own views on, [a] deeply personal question, people will think about it, work their way through it.
"Obviously President Obama has and he's announced a decision."
There is currently same-sex marriage legislation before Parliament, to which both Opposition Leader Tony Abbott and Ms Gillard have signalled their opposition.
But Greens Senator Sarah Hanson-Young says if Mr Obama can support marriage equality, then so should Ms Gillard.
"The majority of people in the United States support marriage equality. Their president does," Senator Hanson-Young said.
"The majority of Australians support marriage equality, yet our Prime Minister doesn't.
"It's time for our political leaders to accept that the Australian people have it right on this one, and currently Julia Gillard and Tony Abbott are on the wrong side of history."
Former Greens leader Bob Brown said it took enormous courage for a political leader to say they have changed their mind.
"His statement is just brilliant and it's going to light the candle for those who are still back in the darkness of wanting this discrimination, and help them to make the change as he's done," Senator Brown said.
Mr Obama, who had previously backed strong protections for gay and lesbian couples, said his position had evolved partly after talking to his daughters, Malia and Sasha, who had some friends with same-sex parents.
"There have been times where Michelle and I have been sitting around the dinner table and we're talking about their friends and their parents, and Malia and Sasha, it wouldn't dawn on them that somehow their friends' parents would be treated differently," he said in the interview.
"It doesn't make sense to them, and frankly, that's the kind of thing that prompts a change in perspective."