McKellar, 39, announced the news on Twitter, writing: "So...I broke a rib in yesterday's #DWTS rehearsal. Trying my best to pull thru and could use your prayers!! #wanttokeepdancing."
It's unclear whether she will be able to compete next week or whether she will be forced to take a break from the dancing competition.
She's not the only one to battle an injury this season. Actor Billy Dee Williams was forced to leave the show earlier this season due to chronic back pain.
"Full House" actress Candace Cameron Bure, NeNe Leakes and Big Time Rush singer James Maslow are among those still in the competition.