Two studies investigate how science, technology, engineering, and math (STEM) professors’ fixed mindsets—the belief that intelligence is fixed and unchangeable—may induce stereotype threat and undermine women’s performance. In an experiment ( N = 217), we manipulated professors’ mindset beliefs (fixed vs. growth) within a course syllabus. While both men and women perceived the fixed mindset professor to endorse more gender stereotypes and anticipated feeling less belonging in the course, women reported these effects more than men. However, only for women did this threat undermine performance. In a 2-year longitudinal field study (884 students enrolled in 46 STEM courses), students who perceived their professor to endorse a fixed (vs. growth) mindset thought the professor would endorse more gender stereotypes and experienced less belonging in those courses. However, only women’s grades in those courses suffered as a result. Together, these studies demonstrate that professors’ fixed mindset beliefs may trigger stereotype threat among women in STEM courses.