Frontier won't be the first city to operate a nonstop to Denver from Louisville.
In all Frontier is expanding to 20 new cities and will add 85 flights.
Frontier now operates a fleet of 70 new Airbus A320-family narrow-body airliners that is expected to grow to more than 100 planes by 2020.
Frontier carried more than 575,000 passengers to and from Miami in 2016, making it MIA's seventh-busiest passenger airline.
For many years, people thought that a low-cost carrier wouldn't make it here in Miami and I think Frontier and other low-cost carriers here have proved them wrong.
The Denver-based carrier announced the four weekly nonstop flights on Tuesday.
Governor Gina Raimondo was in attendance for the announcement Tuesday. It's unknown if the new flights are related to the announcement. "After all is said and done, this is going to have a tremendous amount of awards to everybody".
The airport has added services from three new airlines and nonstop flights to nine destinations in the past 12 months, according to airport officials. Frontier launched into the Miami market in 2014 with service to five USA cities, and soon expanded to seven US destinations with 54 weekly departures in 2015.
The airline said the expansion will almost double nonstop routes by the summer of 2018 and total routes will more than double by then to over 1,000.
The new flights to Cincinnati, Orlando and San Jose will be seasonal and will begin next spring. The airport has also welcomed new worldwide service from Volaris in that time.
Frontier is said to have the second lowest average rate per plane ticket in the country, only behind Spirit Airlines but better than Allegiant.