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

Return flights

16 Sep - 24 Sep	Frontier Airlines
04 Sep - 10 Sep	Frontier Airlines
25 Sep - 28 Sep	Frontier Airlines
25 Sep - 03 Oct	Frontier Airlines
13 Sep - 16 Sep	Frontier Airlines
04 Sep - 09 Sep	Frontier Airlines
04 Sep - 08 Sep	Frontier Airlines
30 Sep - 14 Oct	United
02 Sep - 08 Sep	Frontier Airlines
27 Sep - 11 Oct	United
25 Sep - 03 Oct	United	direct
25 Sep - 09 Oct	United	direct
16 Sep - 30 Sep	United
15 Sep - 29 Sep	United
17 Sep - 01 Oct	United
23 Sep - 07 Oct	United
24 Sep - 08 Oct	United
29 Sep - 13 Oct	United
09 Sep - 23 Sep	United
26 Sep - 10 Oct	United	direct

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 228€

Return flights