The present study aimed to investigate students’ mathematical beliefs and was conducted in two phases using both quantitative and qualitative methods. In the quantitative phase, a descriptive–survey design was used, and in the qualitative phase, thematic analysis was employed. The statistical population consisted of sixth-grade female students in public schools in District 13 of Tehran. In the quantitative phase, 135 students were selected through cluster random sampling, and in the qualitative phase, 9 students were selected through purposive sampling. A mathematical beliefs questionnaire was used in the quantitative phase, and semi-structured interviews were used in the qualitative phase. The quantitative results showed that students valued mathematics, and their beliefs about mathematics and about themselves as mathematics learners (3.33) were above the average (3). In the qualitative phase, the initial code “teacher’s role and teaching method,” the axial code “the nature and characteristics of mathematics,” and the main theme “the nature of mathematics and understanding it” were observed more frequently than the others. Accordingly, most students’ mathematical beliefs appeared to be a mixture—seemingly positive but combined with positive, negative, and instrumental attitudes. They viewed mathematics as conceptual, challenging, applicable, and enjoyable, yet sometimes boring, and believed that success in mathematics is possible through effort, effective teaching, family and teacher support, a positive classroom climate, and positive peer behavior. The findings emphasize the need for simultaneous attention to cognitive and affective dimensions.