Over recent decades, there has been a significant increase in postsecondary STEM education among autistic individuals. Using data from the High School Longitudinal Study of 2009, this study examined the STEM pathways of autistic students, emphasizing key determinants like proximal context, self-efficacy, and outcome expectations within the framework of social cognitive theory. The results revealed that despite a lower college attendance rate, autistic students displayed a pronounced inclination for STEM majors, particularly in the fields of science, engineering, and mathematics. Notably, autistic students who pursue higher education tend to exhibit increased levels of self-efficacy and anticipate more positive outcomes within STEM disciplines. However, the levels of both constructs in mathematics had decreased by the 11th grade. Nonetheless, STEM self-efficacy played a significant role in influencing outcome expectations and major choices, with this relationship being more pronounced among autistic students. For autistic students, their choice of a STEM major was influenced by their self-efficacy, as well as factors like race and gender. On the other hand, for non-autistic students, their proximal context was an additional determinant in their decision. Insights gained from this research can inform educational strategies aimed at facilitating the participation of autistic individuals in postsecondary STEM education and related career paths.