São Valentim do Sul is a municipality in the state of Rio Grande do Sul, in southern Brazil. It lies in the northern part of the state, in a region of small municipalities settled largely by descendants of Italian and German immigrants.
The municipality was established in 1992, when its territory was separated from the neighbouring municipality of Dois Lajeados. As of 2020, its estimated population was 2,245, making it one of the smaller municipalities in Rio Grande do Sul.