Cain, the former Godfather's Pizza CEO, defended his tax plan, saying his proposal is bold and will help create jobs.
Letterman joked that the title of Cain's economic plan appears to be a toll free number.
"You dial 9-9-9, you get a free pizza," Letterman said.
"Instead of a free pizza, when you dial 9-9-9, you're going to be able to get a job," Cain retorted.
Cain's interview airs Friday.