Frontier is the second US airline to require passengers to wear masks or face coverings on flights beginning May 8.
"This new measure is aligned with CDC recommendations and those of many municipalities within the US that include wearing a face covering when out in public," Frontier CEO Barry Biffle said in a press release.
Customers must wear masks when they're around Frontier ticket counters and gate areas, as well as during flights. Young children will be exempt from the policy.
JetBlue announced a similar policy on Tuesday.
On Monday, American Airlines said its flight attendants would be required to wear face masks during every mainline and regional flight starting May 1.