The article outlines the development and initial validation of a Social Vulnerability Index (SVI) including five social determinants of risk to health and demonstrates its application in the financing of primary care by the Unified Health System (SUS) in the State of Rio Grande do Sul. Municipal indicators of vulnerability relating to poverty and population dispersion were obtained from the 2010 population census of the Brazilian Institute of Geography and Statistics. Both exploratory and confirmatory factor analysis suggests that the five items can constitute a reliable and acceptable measurement scale. The SVI-5 was then generated based on the first main component, measuring municipal inequalities in social vulnerability relating to poverty and population in the territory in Z-scores. The external validity of SVI-5 was examined in relation to health outcomes using DATASUS 2007-2011 data, revealing that infant mortality and hospitalizations for conditions treatable by primary care are greater in more vulnerable municipalities The results suggest that the SVI-5 is a valid measure of inequalities in social vulnerability between municipalities, applicable to socially equitable policies in health.