Negatives attitudes toward children with autism are an important barrier to school inclusion. Despite the increasing amount of research, no psychometrically sound scale reliably measures these attitudes in young students. Our aim was to develop and validate a tool (Children’s Attitudes Toward Autism Questionnaire) to evaluate attitudes (three dimensions) of students in elementary school toward peers with autism. Elementary school students ( N = 204) completed the Children’s Attitudes Toward Autism Questionnaire and two other scales assessing behavioral intentions toward peers with mental disability (Shared Activities Questionnaire-B) and familiarity with disability and autism. The confirmatory factor analysis first showed a satisfactory model fit ( χ 2 /df = 3.65, root mean square error of approximation = 0.114, adjusted goodness of fit index = 0.970, standardized root mean square residual = 0.067, comparative fit index = 0.967, and non-normed fit index = 0.963) and internal consistency for each dimension was good (>0.70). Second, attitudes were more positive in girls, older children, and children familiar with disability (but not with autism). The behavioral dimension of the Children’s Attitudes Toward Autism Questionnaire had the strongest association with the Shared Activities Questionnaire-B. The Children’s Attitudes Toward Autism Questionnaire is the first scale (1) to assess the three dimensions of attitudes toward autism (2) among children (from the age of 6 years old) and (3) to show good psychometric quality. With the Children’s Attitudes Toward Autism Questionnaire, researchers can reliably measure children’s attitudes toward peers with autism, which is especially necessary when evaluating the effectiveness of much-needed anti-stigma programs. Lay abstract Research has shown that negative attitudes toward a different child can appear very early in development. Unfortunately, these negative attitudes are one of the most important barriers to the school inclusion of children with autism. Despite the increasing amount of research, no tool reliably measures these attitudes among young students. The objective of this study was to develop and validate a questionnaire (Children’s Attitudes Toward Autism Questionnaire) to evaluate attitudes of students in elementary school toward their peers with autism. Elementary school students ( N = 204) completed the Children’s Attitudes Toward Autism Questionnaire and two other scales assessing behavioral intentions toward peers with a mental disability (Shared Activities Questionnaire-B) and familiarity with disability and autism. Results first showed that the Children’s Attitudes Toward Autism Questionnaire reliably measured the concept of attitude through three sub-dimensions (namely, the cognitive, affective, and behavioral dimensions). Second, analyses confirmed that the Children’s Attitudes Toward Autism Questionnaire corresponds with previous knowledge on this topic, namely, that attitudes were more positive in girls, older children, and children familiar with disability. In conclusion, the Children’s Attitudes Toward Autism Questionnaire is the first scale (1) to assess all the dimensions of attitudes toward autism among elementary school children (from the age of 6 years old) and (2) to show theoretical and statistical relevance. From now on, the Children’s Attitudes Toward Autism Questionnaire can be used to assess attitudes of young children toward their peers with autism. This is an important step forward, in particular for evaluating the effects of anti-stigma programs that are increasingly implemented in schools.