The latest action follows concern over the danger posed by the Delta variant, which has rapidly spread throughout the United States.
"As we continue to watch the rapid increase of new COVID-19 cases across the United States caused by the Delta variant, I am concerned for the well-being of our team members, their families and friends," said Barry Biffle, Frontier's president and CEO. "Safety is of the utmost importance at Frontier and we need to take every step possible for us to keep our teams safe, protect the operation and protect our passengers."
"The time has come to do what we can to help put an end to COVID-19," Biffle added.
The airline made it clear that employees that choose not to or are unable to get vaccinated will be asked to provide proof of a negative test on a regular basis.
The airline has asked union leaders to help them establish testing protocols for employees, the company announced in its statement.
Biffle noted that "the vast majority" of employees have already gotten vaccinated, but the policy is a push to "further increase" that percentage among the workforce.
Frontier joins United Airlines, which also announced on Friday a mandate for employee vaccinations. The United guidelines are the same as Frontier’s: An Oct. 1 deadline or face "regular" testing.
"We know some of you will disagree with this decision to require the vaccine for all United employees," CEO Scott Kirby and President Brett Hart told employees Friday. But he added, "The facts are crystal clear: Everyone is safer when everyone is vaccinated."
United, which has 67,000 employees in the United States, has required new hires to be vaccinated since mid-June. Unvaccinated workers are required to wear face masks at company offices.
The Associated Press contributed to this report.