Use the "Price (lowest to highest)" filter on Expedia to find cheap Frontier Airlines round-trip flights from $237 from STL to ISP. Affordable one-way fares from $129 are also on offer.**These prices were available on Expedia within the past 7 days, for flights departing within the next year. Prices and availability are subject to change. Additional terms apply.

For cheap Frontier Airlines fares from Lambert-St. Louis Intl. Airport (STL) to MacArthur Airport (ISP), start searching one to three months before your intended departure. Securing domestic flights 34 to 86 days in advance could save you 25% compared to booking just 6 to 12 days ahead. The day you book can also influence the overall cost. Sunday is generally the most affordable, while Monday is often the most expensive for domestic flights. You can save 6% on average by booking your flight on a Sunday.**Ideal booking window recommendation is based on average round-trip ticket prices for January through November for 2024. Best day of the week to book recommendation is based on average round-trip ticket prices for January through October 2024. All data sourced from ARC's global airline sales database.

To find the earliest departure from STL to ISP with Frontier Airlines, filter your search by "Morning" and "Early Morning." Booking an early flight usually means you'll experience fewer cancellations.**Most reliable time of the day recommendation is based on January through August 2024 flight status data sourced from OAG's global flight data platform.

A rewarding loyalty program and the ability to rack up points through Frontier Miles are the start of the reasons to fly from STL to ISP with Frontier Airlines. Plus, by joining Expedia's free One Key™ program, you can receive OneKeyCash™* in addition to Frontier Airlines's loyalty program earnings. You can put it toward your future trips.*OneKeyCash™ disclaimer - OneKeyCash™ is not redeemable for cash and can only be used on Expedia, Hotels.com and Vrbo.