AirHint found the following cheapest one-way flights from St. Louis (STL) to Denver (DEN) in September 2025 (see full year).

One-Way flights

26 Sep	Frontier Airlines
19 Sep	Frontier Airlines
26 Sep	Frontier Airlines	direct
20 Sep	Frontier Airlines	direct
16 Sep	Frontier Airlines	direct
14 Sep	Frontier Airlines
23 Sep	United	direct
11 Sep	Frontier Airlines	direct
19 Sep	Frontier Airlines	direct
13 Sep	Frontier Airlines	direct

Also check the deals for return flights from St. Louis to Denver in September.

The deals are updated regularly. The prices may go up as tickets are purchased by other travellers. Only the cheapest flights are offered. Select a price to book and secure these cheap prices.

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Cheapest one-way flights from St. Louis to Denver in September from $85

One-Way flights