DeGeneres, 61, hosted Cardi B, 27, on the talk show while dressed up as "Cardi E" for Halloween.
DeGeneres said her costume, which consisted of a black sequin outfit, blonde wig and fake breasts and fingernails, was an homage to Cardi B's character in the movie Hustlers.
"I like that yours are softer than mine," Cardi B said while touching DeGeneres' fake chest. "I should be mad at my doctor."
Cardi B helped DeGeneres to twerk by encouraging the television personality to pop higher.
"I'll give you 20 dollars. I'd buy that dance from you," Cardi B said.
In her interview, Cardi B said she planned to go trick-or-treating with Kulture, her 15-month-old daughter with Offset.
"I'm going to dress up as Moana because my baby's dressing up as Moana," she said. "So I'm trying to go trick-or-treating for me because she can't eat candies. That's the fun thing about being a mom."
Cardi B shared a photo earlier in the week of herself dressed up as a sexy nurse.
Cardi B last released the single "Yes" with Fat Joe and Anuel AA in September. News broke last week that she will star in the new Fast and the Furious movie, Fast 9.