This article presents the design and validation of a scale to assess Normative Beliefs About Prosociality (CNPROS), taking as a model the Normative Beliefs About Aggression Scale, NBAGGS (Huesmann & Guerra, 1997). The validation was developed in several phases during 2012 and 2013, with a convenience sample of 762 4th, 5th and 6th graders from 4 private subsidized schools located in 4 different neighborhoods in Santiago, Chile. Analyses were carried out with 4 different samples of students from different grades and schools. Through exploratory and confirmatory factor analyses, the structure and internal consistency of the scale were shown. Its direct correlations with empathy (for girls) and prosocial behavior (where significant gender differences cannot be discerned) and inverse correlations with NBAGGS and aggression showed concurrent validity. An analysis of variance among 6th graders showed that girls obtained a higher average score than boys in CNPROS. A 9 items scale is finally presented, which may help the diagnosis, research, and intervention in school contexts.