"He's said up until now that his position was evolving. I think most of us thought that at some point he would endorse the legalization, that is the equalization of same-sex marriage, and if that's what he's got to do I respect him for having the guts to say what he believes," said Lieberman on MSNBC's "Andrea Mitchell Reports."
This was the first interview for the president since Vice President Biden said Sunday that he is "absolutely comfortable" with men marrying men and women marrying women.
Following Biden's remarks, Secretary of Education Arne Duncan also voiced his support for same-sex marriage.
The issue was further brought into the spotlight this week when voters overwhelmingly approved a referendum Tuesday in North Carolina banning gay marriage.
Obama's reelection campaign said he was "disappointed" that North Carolina voters approved at the constitutional amendment.
"The president has long opposed divisive and discriminatory efforts to deny rights and benefits to same-sex couples," Obama campaign spokesman Cameron French said in a Tuesday statement on the vote over Amendment 1.
Lieberman, who spoke before Obama's remarks, acknowledged that coming out in support of gay marriage in an election year is a controversial position for the president to take, but noted the public support has grown over the years.
He said he personally supports civil unions, but believes marriage should be reserved for the union of a man and woman.
"My guess is that within the Democratic Party, most people support legalizing same-sex marriage," he said.