"I'm pregnant!" UsMagazine.com said the 53-year-old guest co-host joked with the studio audience on Monday's edition of the chat show.
"No! No! But my daughter-in-law is!" she said, explaining her son Stephen Craig and daughter-in-law Claire Olds are expecting their first child.
She did not reveal the baby's gender or anticipated due date.
"I am so excited I'm going to be a grandma," Osmond said. "I just can't wait. We went in for my son -- for his birthday -- he turned 30, which is amazing 'cause I'm 29. ... He blew out the candles and then blew me away and said, 'You're gonna be a grandma!' It was really fun."