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

Return flights

28 Sep - 07 Oct	Icelandair	direct
23 Sep - 08 Oct	Icelandair	direct
30 Sep - 07 Oct	Icelandair	direct
25 Sep - 09 Oct	Icelandair	direct
21 Sep - 02 Oct	Air France
30 Sep - 08 Oct	Icelandair	direct
23 Sep - 30 Sep	Air France
23 Sep - 28 Sep	Delta
23 Sep - 18 Oct	Air Canada
23 Sep - 29 Sep	Air Canada
22 Sep - 29 Sep	Air Canada
23 Sep - 18 Oct	Icelandair	direct
26 Sep - 03 Oct	Lufthansa
21 Sep - 02 Oct	Icelandair	direct
22 Sep - 29 Sep	Icelandair	direct
19 Sep - 26 Sep	Icelandair	direct
23 Sep - 30 Sep	Icelandair	direct
21 Sep - 22 Sep	Icelandair
23 Sep - 28 Sep	Icelandair	direct
21 Sep - 22 Sep	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.

Book a cheap hotel in Keflavik

Cheap car rent in Keflavik

Cheapest return flights from Seattle to Keflavik in September from $427

Return flights