The 'Good Girl' singer, who is married to NHL star Mike Fisher, told The Independent that it's unfair that members of the LGBT community are unable to wed their partners in many states.
"I can't imagine how that must feel. I definitely think we should all have the right to love, and love publicly, the people that we want to love," Underwood commented.
Underwood went on to reveal that her faith actually helped form her opinion on same-sex unions.
"Our church is gay-friendly. Above all, God wanted us to love others. It's not about setting rules, or [saying] 'everyone has to be like me'. No," she insisted.
"We're all different. That's what makes us special. We have to love each other and get on with each other. It's not up to me to judge anybody."
Underwood joins a growing list of celebrities who have publicly endorsed same-sex marriage, including Brad Pitt, Will Smith and Kristen Bell.
US President Barack Obama announced his own support for the cause in May after consulting family, friends and military advisors.
Watch Carrie Underwood's 'Good Girl' music video below: