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

Return flights

16 Sep - 23 Sep	Air Canada
26 Sep - 09 Oct	Icelandair	direct
16 Sep - 24 Sep	Icelandair	direct
30 Sep - 08 Oct	Icelandair	direct
14 Sep - 23 Sep	Air Canada
07 Sep - 16 Sep	Air France
13 Sep - 18 Sep	Air France
01 Sep - 09 Sep	Delta
09 Sep - 17 Sep	Air France
08 Sep - 16 Sep	Delta
16 Sep - 23 Sep	Icelandair	direct
14 Sep - 23 Sep	Icelandair	direct
27 Sep - 04 Oct	Air Canada
26 Sep - 03 Oct	Air Canada
30 Sep - 11 Oct	Air France
27 Sep - 10 Oct	Icelandair	direct
24 Sep - 03 Oct	Air Canada
26 Sep - 03 Oct	Icelandair	direct
30 Sep - 11 Oct	Icelandair	direct
24 Sep - 03 Oct	Icelandair	direct

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 361€

Return flights