The 33-year-old singer was performing her set Wednesday night at the Planet Hollywood Resort and Casino when, after a brief jump, she landed hard on her left ankle and twisted it.
Video from the incident indicated that she was in significant pain as she performed, but she continued to play through the set.
"Thank you for all of the sweet wishes," she wrote on Twitter. "Had a little scare on stage tonight with my ankle but I'm ok!"
Later, though, Spears posted to Instagram, noting that her doctors had advised canceling two shows to give her time to recover.
Dates for rescheduling of the missed concerts have not been announced.
In a joint statement, Spears and Planet Hollywood said they "apologize for any inconvenience this causes ticket holders."
Spears has been performing at her Vegas residency since 2013.
April was a tough month for performance-related injuries in pop as singer Florence Welch of Florence + The Machine fractured her foot during a first-weekend performance at the