| Official Name: | Ust-Sema |
| Native Name: | Усть-Сема |
| Settlement Type: | Village |
| Pushpin Map: | Russia Altai Republic#Russia |
| Subdivision Type: | Country |
| Subdivision Name: | Russia |
| Subdivision Name1: | Altai Republic |
| Subdivision Name3: | Chemalsky District |
| Subdivision Type4: | Municipality |
| Utc Offset1: | +7:00 |
| Coordinates: | 51.6333°N 130°W[1] |
Ust-Sema (Russian: Усть-Сема; Southern Altai: Себи-Оозы, Sebi-Oozı) is a rural locality (a selo) in Cheposhskoye Rural Settlement of Chemalsky District, the Altai Republic, Russia. The population was 402 as of 2016.[2] There are 15 streets.
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.[3]