Tosu (Russian: Тосу; Yakut: Халбаакы) is a rural locality (a selo), the administrative centre of and one of two settlements, in addition to Starovatovo, in Khalbakinsky Rural Okrug of Vilyuysky District in the Sakha Republic, Russia. It is located 18 kilometers (11 mi) from Vilyuysk, the administrative center of the district. Its population as of the 2010 Census was 665, of whom 325 were male and 340 female, down from 731 as recorded during the 2002 Census.