US President Barack Obama says he thinks same-sex couples should be able to marry.
Mr Obama says his view is based on conversations with his own staff members and openly gay and lesbian service members, and conversations with his family.
In recent days, Vice-President Joe Biden and cabinet member Arne Duncan had expressed support for gay unions.
The president had been under under mounting pressure to clarify his position on the issue and made his remarks in an ABC News interview.
"It is important for me to go ahead and affirm that I think same-sex couples should be able to get married," he said.
Mr Obama has become the first US president to publicly back gay marriage.
Gay rights groups in the United States have praised Mr Obama for supporting same-sex marriage.
Gay marriage is legal in eight states as well as in the capital, Washington DC, and Mr Obama says he believes it should remain an issue for each state to legislate on.
Democratic Party congressman Barney Frank who plans to marry his male partner later this year, says it is a special day for him.
"It means a great deal to me and I must say it makes me feel even better about my country."
However Republican Mitt Romney, Mr Obama's likely rival in this year's presidential election, has come out against gay marriage in response to the news that the president supports it.