The integration of academic disciplines such as science and mathematics (S&M) can help foster students’ interest and sense of self-efficacy (S-E). However, few questionnaires appear to have been developed to measure these perceptual variables in this type of context. Nor do any appear to exist that are suitable for elementary students from low-income and multicultural schools. This article reports the results of the validation process of such a questionnaire including six scales conducted among 1,553 grade 5 or 6 students from schools in the Montreal area in Canada. In its final version, the Sciences and Mathematics Integration Questionnaire (SMIQ) measures six perceptual variables: science S-E, mathematics S-E, interest in science, interest in mathematics, interest in integrated S&M activities and perception of the link between sciences and mathematics. To our knowledge, the last two variables have never been measured and the proposed scales to measure them are new.