Ust-Sema (Russian: Усть-Сема; Altay: Себи-Оозы, Sebi-Oozı) is a rural locality (a selo) in Cheposhskoye Rural Settlement of Chemalsky District, the Altai Republic, Russia.

Ust-Sema is located on the right bank of the Katun River, 37 km northwest of Chemal (the district's administrative centre) by road.

Verkh Barangol is the nearest rural locality.

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