The 39-year-old actress and author took to Twitter.com on Friday (25Apr14) and asked her supporters to pray for her quick recovery.
She wrote, "So...I broke a rib in yesterday's #Dwts rehearsal. Trying my best to pull thru and could use your prayers!! #wanttokeepdancing."
It is not yet known if MCKellar will be able to dance her way through the pain or be forced to sit out the next round of live shows next week (begs28Apr14).
Earlier this season, Star Wars hero Billy Dee Williams pulled out of the ballroom competition show due to a chronic back problem.
Other celebrities still in the competition for season 18 of Dancing with the Stars include former Full House actress Candace Cameron Bure, Glee star NeNe Leakes and Big Time Rush singer James Maslow.