União da Serra is a municipality in the state of Rio Grande do Sul, in southern Brazil. It is one of the many small municipalities that make up the state's interior, and it is located within the Rio Grande do Sul region rather than in any other Brazilian state that might share a similar place name.
The municipality had an estimated population of 1,118 as of 2020, making it a very small community by Brazilian standards. That figure reflects its status as a minor administrative unit within Rio Grande do Sul.