Robert Jeffress, senior pastor at First Baptist Church in Dallas, endorsed Perry at the Values Voters Summit, introducing him as "a proven leader, a true conservative, and a committed follower of Christ."
After his remarks, Jeffress told reporters that Perry's religion is different from Romney's.
"Rick Perry's a Christian. He's an evangelical Christian, a follower of Jesus Christ," Jeffress said. "Mitt Romney's a good moral person, but he's not a Christian. Mormonism is not Christianity. It has always been considered a cult by the mainstream of Christianity."
Romney is a member of the Church of Jesus Christ of Latter-day Saints, whose members are commonly called Mormons.
Perry's campaign said the Texas governor disagrees with Jeffress.
"The governor does not believe Mormonism is a cult," said spokesman Mark Miner.
Perry's campaign said that Values Voters Summit organizers, not the campaign, chose Jeffress to introduce the Texas governor. Perry's campaign did not immediately respond to inquiries about whether the governor had agreed to have Jeffress introduce him.
Jeffress had made similar comments about Romney before, during the former Massachusetts governor's first presidential run in 2008.
"Mitt Romney is a Mormon, and don't let anybody tell you otherwise. Even though he talks about Jesus as his lord and savior, he is not a Christian," Jeffress said in a 2007 sermon. "Mormonism is not Christianity. Mormonism is a cult. And just because somebody talks about Jesus does not make them a believer."
In that sermon, Jeffress said he was frustrated that some religious leaders had backed Romney anyway. "What really distresses me is some of my ministerial friends, and even leaders in our convention, say, `Well, he talks about Jesus, we talk about Jesus, what's the big deal?' It is a big deal."