AirHint found the following cheapest return flights from Denver (DEN) to Keflavik (KEF) in September 2025 (see full year).

Return flights

27 Sep - 04 Oct	Frontier Airlines
22 Sep - 18 Oct	Frontier Airlines
17 Sep - 21 Sep	United
19 Sep - 26 Sep	Frontier Airlines
20 Sep - 24 Sep	Frontier Airlines
21 Sep - 26 Sep	Frontier Airlines
25 Sep - 28 Sep	Frontier Airlines
18 Sep - 25 Sep	Frontier Airlines
26 Sep - 28 Sep	Frontier Airlines
18 Sep - 27 Sep	Frontier Airlines
19 Sep - 22 Sep	Frontier Airlines
18 Sep - 20 Sep	Air Canada
27 Sep - 04 Oct	Icelandair	direct

Also check the deals for one-way flights from Denver to Keflavik 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 return flights from Denver to Keflavik in September from 476€

Return flights