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

Return flights

29 Sep - 02 Oct	Fly Play
28 Sep - 02 Oct	Wizz Air
26 Sep - 29 Sep	Fly Play
26 Sep - 29 Sep	Fly Play	direct
21 Sep - 28 Sep	Fly Play
28 Sep - 02 Oct	Icelandair	direct
18 Sep - 21 Sep	Wizz Air Malta
21 Sep - 28 Sep	Icelandair	direct
19 Sep - 21 Sep	Fly Play
19 Sep - 21 Sep	Fly Play	direct
18 Sep - 21 Sep	Icelandair	direct

Also check the deals for one-way flights from Keflavik to Lisbon 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 Keflavik to Lisbon in September from 293€

Return flights