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

Return flights

11 Sep - 20 Sep	Air Canada
10 Sep - 23 Sep	Air Canada
09 Sep - 16 Sep	Hawaiian Airlines
28 Sep - 07 Oct	WestJet
13 Sep - 23 Sep	United
25 Sep - 09 Oct	Air Canada
05 Sep - 12 Sep	Air Canada
03 Sep - 10 Sep	Air Canada
29 Sep - 07 Oct	WestJet
04 Sep - 11 Sep	Air Canada
10 Sep - 17 Sep	Air Canada
11 Sep - 18 Sep	Air Canada
06 Sep - 16 Sep	Air France
24 Sep - 01 Oct	Alaska Airlines
23 Sep - 01 Oct	Alaska Airlines
22 Sep - 01 Oct	Alaska Airlines
25 Sep - 02 Oct	Alaska Airlines
29 Sep - 09 Oct	Air France
28 Sep - 09 Oct	Air France
30 Sep - 06 Oct	Alaska Airlines

Also check the deals for one-way flights from Seattle 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 Seattle to Keflavik in September from $476

Return flights