"Marriage is between one man and one woman. The President took an oath to uphold the Constitution & needs to defend our laws, including DOMA," Barbour tweeted.
Barbour, a potential 2012 presidential candidate, joins other prominent conservatives, including former Sen. Rick Santorum (R-Pa.) and Rep. Michele Bachmann (R-Minn.), in opposing President Obama's decision.
The president on Wednesday ordered the Department of Justice to stop defending in court the Defense of Marriage Act, which limits same-sex unions.
But some of the most commonly mentioned possible contenders for the Republican nomination in 2012, such as former Alaska Gov. Sarah Palin and former Minnesota Gov. Tim Pawlenty, have yet to weigh in on Obama's decision.