Increasing the number of science, technology, engineering, and mathematics (STEM) degrees is a national priority and one way to promote the socioeconomic mobility of students from low-income families. Prior research examining why students do not complete STEM majors often points to students' lack of academic preparation, preferences for non-STEM majors, or lack of information about the value of STEM. This paper uses a randomized experiment to investigate an alternative explanation, that some students lack the financial resources to succeed in demanding majors. In a control group of university students from low-income families, 18.6% of students had declared a STEM major by their third year of college. In a treatment group who were offered additional need-based grant aid upon entering college, 26.5% of students declared a STEM major. Among students who had graduated within six years after entering college, 12.2% of control group graduates had earned a STEM degree compared to 20.2% of treatment group students. Need-based grants thus appear to have the potential to increase the share of low-income students studying and earning degrees in STEM.