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

Return flights

04 Dec - 11 Dec	SWISS
06 Dec - 15 Dec	British Airways
05 Dec - 15 Dec	British Airways
04 Dec - 11 Dec	Icelandair	direct
06 Dec - 15 Dec	Icelandair	direct
05 Dec - 15 Dec	Icelandair	direct
09 Dec - 16 Dec	Icelandair	direct
24 Dec - 31 Dec	Icelandair	direct
14 Dec - 21 Dec	Icelandair	direct
29 Dec - 05 Jan	Icelandair	direct
19 Dec - 26 Dec	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 $637

Return flights