As in every field, interest is an important factor in learning mathematics. Determining students' interest in mathematics and implementation of activities according to the results could positively influence their achievement in this area. In Turkey, research into student interest is generally limited to scientific and vocational interest, with a limited number of studies having been conducted on the interest of middle school students with mathematics. Thus, this study was conducted to elicit middle school students' mathematical interest and designed as a mixed model. Within the scope of the study, the "Mathematical Interest Scale for Middle School Students" was developed by the researchers, taking into account the theoretical frame of interest for quantitative data and the relevant literature. In addition, a semi-structured interview form was prepared to elicit qualitative data to obtain in-depth knowledge about the factors affecting the middle school students' interest in mathematics. The quantitative and qualitative stages of the study were conducted with 1671 students (827 female, 844 male) and 17 students (10 female, 7 male), respectively. The results revealed that seventh-grade students had a high level and eighth-grade students had a moderate level of interest in mathematics. Furthermore, female students' interest in mathematics was at a high level and that of male students was at a moderate or high level. The students reported the factors that affected their interest in mathematics as themselves, their family, and their teachers.