McKellar, 39, and her partner have consistently remained at the top of the leader board during Season 18 of the ABC series, reported Us magazine.
"So... I broke a rib in yesterday's DWTS rehearsal. Trying my best to pull thru and could use your prayers!! Had rehearsal today.. It's tough dancing with a broken rib," she posted on Twitter.
The dancing duo earned a 39 out of 40 for their 'Beauty and the Beast' Disney routine. Last week, they earned a 36 out of 40.