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

Return flights

03 Sep - 10 Sep	Delta
28 Sep - 10 Oct	Frontier Airlines
04 Sep - 11 Sep	Delta
19 Sep - 26 Sep	Porter Airlines (Canada) Ltd
08 Sep - 13 Sep	Porter Airlines (Canada) Ltd
08 Sep - 14 Sep	Porter Airlines (Canada) Ltd
13 Sep - 20 Sep	Frontier Airlines
19 Sep - 11 Oct	Porter Airlines (Canada) Ltd
22 Sep - 28 Sep	Porter Airlines (Canada) Ltd
11 Sep - 15 Sep	Porter Airlines (Canada) Ltd
10 Sep - 17 Sep	Frontier Airlines
09 Sep - 17 Sep	Delta
27 Sep - 05 Oct	Delta
20 Sep - 11 Oct	Virgin Atlantic
10 Sep - 14 Oct	Delta

Also check the deals for one-way flights from Chicago 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 Chicago to Lisbon in September from 643€

Return flights