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

Return flights

30 Sep - 07 Oct	Icelandair	direct
24 Sep - 01 Oct	Icelandair	direct
06 Sep - 12 Sep	Icelandair	direct
02 Sep - 08 Sep	Icelandair	direct
03 Sep - 08 Sep	Icelandair	direct
22 Sep - 29 Sep	Icelandair	direct
06 Sep - 13 Sep	Icelandair	direct
16 Sep - 23 Sep	Icelandair	direct
25 Sep - 09 Oct	Icelandair	direct
10 Sep - 23 Sep	Icelandair	direct
11 Sep - 17 Sep	Air France
13 Sep - 22 Sep	Air France
11 Sep - 18 Sep	Air Canada
26 Sep - 03 Oct	Icelandair	direct
28 Sep - 05 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 340€

Return flights