USA Today said Underwood, 29, was performing on stage Saturday night when she spotted a boy in the audience, holding up a sign that said, "Will you be my first kiss?"
She invited the fan, identified only as Chase, on stage and asked, "How are we gonna do this?" the newspaper reported.
"Lip to lip," the boy said.
Underwood gave him a quick peck on the lips and told the crowd, "The only time that's going to happen."
She also joked with the boy about how her husband, professional hockey player Mike Fisher, was in the building somewhere "and might be looking for you later," USA Today said.