Educational science has consistently examined what drives adolescents to aspire to occupations in math-related fields -- and which factors, such as gender stereotypes, may drive them away. In this study, we utilized a large longitudinal data set (N = 1,092) to test whether past grades, gender, and the math level of mothers' and fathers' occupations predicted the math-relatedness of German adolescents' occupational aspirations 1 year later in 10th grade controlling for mathematical competence, intelligence, and socioeconomic status, among other factors. First, we examined whether motivational beliefs -- math intrinsic value and math ability self-concept -- mediated these relationships. Second, we used a latent interaction approach to test whether intrinsic value and ability self-concept interacted to predict the math-relatedness of aspirations, an important yet understudied prediction of expectancy-value theories. The intrinsic value of math, but not ability self-concept fully mediated the effect of math grade on aspirations. Gender differences could partly be explained via intrinsic value, but a substantial effect remained. The math level of mothers', but not fathers' occupations was positively associated with the math-relatedness of their children's aspirations, but this effect was not mediated by motivational beliefs. Exploratory analyses examined these relations separately for boys and girls. Regarding our second aim, there was a significant interaction showing that intrinsic value only predicted the math-relatedness of aspirations among students with a high-ability self-concept. This latter finding points to the importance of continuing to investigate not only main effects but also interaction effects within the framework of the situated expectancy-value theory.