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

Return flights

01 Sep - 10 Sep	Aer Lingus
30 Sep - 25 Oct	TAP
18 Sep - 23 Sep	Aer Lingus
23 Sep - 18 Oct	TAP
21 Sep - 26 Sep	Frontier Airlines
11 Sep - 23 Sep	Aer Lingus
24 Sep - 30 Sep	American Airlines	direct
03 Sep - 17 Sep	Frontier Airlines
23 Sep - 30 Sep	jetBlue
23 Sep - 05 Oct	Spirit Airlines
14 Sep - 04 Oct	Frontier Airlines
08 Sep - 04 Oct	Aer Lingus
10 Sep - 20 Sep	Frontier Airlines
17 Sep - 11 Oct	Frontier Airlines
03 Sep - 07 Sep	jetBlue
19 Sep - 28 Sep	Spirit Airlines
29 Sep - 06 Oct	United
30 Sep - 25 Oct	Iberia	direct

Also check the deals for one-way flights from Philadelphia 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 Philadelphia to Lisbon in September from 541€

Return flights