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

Return flights

06 Sep - 12 Sep	Frontier Airlines
06 Sep - 13 Sep	Frontier Airlines
30 Sep - 06 Oct	Frontier Airlines
19 Sep - 23 Sep	Frontier Airlines
19 Sep - 22 Sep	Frontier Airlines
09 Sep - 16 Sep	Frontier Airlines
11 Sep - 14 Sep	Frontier Airlines
03 Sep - 08 Sep	Frontier Airlines
26 Sep - 28 Sep	United	direct
06 Sep - 13 Sep	United	direct
01 Sep - 07 Sep	Southwest Airlines

Also check the deals for one-way flights from Denver 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 Denver to San Juan in September from $257

Return flights