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

Return flights

20 Sep - 23 Sep	Frontier Airlines
15 Sep - 20 Sep	Frontier Airlines
19 Sep - 22 Sep	Frontier Airlines
20 Sep - 25 Sep	Frontier Airlines
11 Sep - 18 Sep	Frontier Airlines
26 Sep - 03 Oct	Southwest Airlines
26 Sep - 29 Sep	Frontier Airlines
27 Sep - 03 Oct	Southwest Airlines

Also check the deals for one-way flights from Chicago 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 Chicago to Keflavik in September from 413€

Return flights