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

Return flights

02 Dec - 09 Dec	Icelandair	direct
03 Dec - 10 Dec	Icelandair	direct
04 Dec - 11 Dec	Icelandair	direct
08 Dec - 15 Dec	Icelandair	direct
05 Dec - 14 Dec	Lufthansa
05 Dec - 12 Dec	Icelandair	direct
03 Dec - 27 Dec	Icelandair	direct
11 Dec - 18 Dec	Icelandair	direct
01 Dec - 08 Dec	Icelandair	direct
09 Dec - 16 Dec	Icelandair	direct
30 Dec - 06 Jan	Icelandair	direct
31 Dec - 07 Jan	Icelandair	direct
29 Dec - 05 Jan	Icelandair	direct
25 Dec - 01 Jan	Icelandair
27 Dec - 03 Jan	Icelandair	direct

Also check the deals for one-way flights from Seattle to Keflavik in December.

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 December from $448

Return flights