AirHint found the following cheapest return flights from Washington (IAD) to San Juan (SJU) in September 2025 (see full year).

Return flights

20 Sep - 28 Sep	Frontier Airlines
19 Sep - 21 Sep	Frontier Airlines
29 Sep - 06 Oct	Frontier Airlines
21 Sep - 25 Sep	United	direct
14 Sep - 18 Sep	United
02 Sep - 11 Sep	United
17 Sep - 24 Sep	United
24 Sep - 26 Sep	United
15 Sep - 22 Sep	Frontier Airlines
19 Sep - 28 Sep	Frontier Airlines
23 Sep - 29 Sep	United
26 Sep - 04 Oct	Frontier Airlines

Also check the deals for one-way flights from Washington to San Juan 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 Washington to San Juan in September from 201€

Return flights