AirHint found the following cheapest return flights from Seoul (GMP) to Ulaanbaatar (UBN) in September 2025 (see full year).

Return flights

06 Sep - 11 Sep	T'way Air
27 Sep - 05 Oct	Korean Air
25 Sep - 29 Sep	T'way Air
12 Sep - 14 Sep	T'way Air
15 Sep - 18 Sep	Air China
03 Sep - 07 Sep	Air China
03 Sep - 10 Sep	Air China
12 Sep - 21 Sep	Air China
08 Sep - 16 Sep	Air China
17 Sep - 27 Sep	Air Busan
16 Sep - 23 Sep	Air China
29 Sep - 04 Oct	Air China
28 Sep - 01 Oct	Air China
28 Sep - 02 Oct	Air China
28 Sep - 12 Oct	Air China
30 Sep - 12 Oct	Air China
30 Sep - 06 Oct	Air China
29 Sep - 02 Oct	Air China

Also check the deals for one-way flights from Seoul to Ulaanbaatar 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 Seoul to Ulaanbaatar in September from 241€

Return flights