Ust-Sema (; ) is a rural locality (a selo) in Cheposhskoye Rural Settlement of Chemalsky District, the Altai Republic, Russia. The population was 402 as of 2016. 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.