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

Return flights

29 Sep - 29 Oct	Frontier Airlines
29 Sep - 13 Oct	Air France
27 Sep - 27 Oct	Frontier Airlines
28 Sep - 28 Oct	Frontier Airlines
23 Sep - 13 Oct	Frontier Airlines
23 Sep - 14 Oct	Frontier Airlines
28 Sep - 14 Oct	Frontier Airlines
23 Sep - 23 Oct	Frontier Airlines
23 Sep - 07 Oct	Frontier Airlines
25 Sep - 09 Oct	Frontier Airlines

Also check the deals for one-way flights from Chicago to Sofia 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 Sofia in September from $692

Return flights