As new initiatives and programs are being increasingly implemented to promote STEM (Science, Technology, Engineering, and Mathematics) interest and participation among U.S. students, the percentage of students who declare a STEM-related major in college continues to lag behind what would be expected based on students' intentions. Such findings underscore the value of understanding why students who are interested in STEM do not pursue a STEM degree. In order to answer that question, the current study developed a multidimensional model of STEM major choice based on academic and non-academic student characteristics. The identification factors to include in the model was based on previous literature findings supporting psychological theories related to major selection. Namely, we focused on the theory of planned behavior and person-environment fit models. The findings support these theories with students' achievement levels, their high school coursework and grades, their major intentions, the certainty of their major intentions, and having measured interest in STEM being related significantly to STEM major choice. The results can help inform initiatives to identify students most likely to enter the STEM pipeline and provide resources and support for this career pathway. Two appendices provide supplemental tables and additional details regarding the calculations of the adjusted odds rations (ORs).