Research in mathematics education has shown that individuals' beliefs play a fundamental role in their responses to and interest in mathematics and their application of these beliefs in real-life situations. Although the literature related to the affective domain in general, and beliefs in particular, has explored the relationship between beliefs and student educational level, there still needs to be a research gap regarding the empirical testing of this variable in the context of secondary mathematics education in Chile. Therefore, the present study, with a quantitative and exploratory methodology, aims to fill this gap by adapting the Mathematics-Related Beliefs Questionnaire MRBQ to describe and analyze students' beliefs in mathematics at two levels of secondary education. The main results of the study indicate that mostly positive beliefs are observed in the three types of beliefs studied: Beliefs about the social context, about self-concept, and mathematics education, highlighting the dimension of beliefs about the social context or classroom environment, in which students showed a highly positive perception of the role of the mathematics teacher.