Castor River South is a local service district and designated place in the Canadian province of Newfoundland and Labrador.
Castor River South is in Newfoundland within Subdivision C of Division No. 9.
As a designated place in the 2016 Census of Population conducted by Statistics Canada, Castor River South recorded a population of 136 living in 50 of its 56 total private dwellings, a change of from its 2011 population of 141. With a land area of , it had a population density of in 2016.
Castor River South is a local service district (LSD) that is governed by a committee responsible for the provision of certain services to the community. The chair of the LSD committee is Gerard Rumbolt.