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

Return flights

30 Sep - 07 Oct	Delta
15 Sep - 24 Sep	Delta
16 Sep - 23 Sep	Delta
08 Sep - 18 Sep	Delta
09 Sep - 16 Sep	Delta
11 Sep - 18 Sep	Porter Airlines (Canada) Ltd
28 Sep - 10 Oct	Delta
29 Sep - 05 Oct	Delta
12 Sep - 23 Sep	Porter Airlines (Canada) Ltd
12 Sep - 27 Sep	KLM
10 Sep - 18 Sep	KLM
11 Sep - 21 Sep	Delta
10 Sep - 20 Sep	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 551€

Return flights