Recent international studies have found that teachers' attitudes, biased against historically marginalized groups, predict lower student achievement in mathematics (e.g., van den Bergh et al., 2010). It is not clear, however, if or how teachers' racial attitudes affect their evaluation of students' mathematical thinking to produce these effects. Using an experimental design, we conducted an online survey to examine the relationship between preservice teachers' (PSTs) racial attitudes and their perceptions of students' mathematical thinking. The survey used comparable videos, with similar mathematics content and student thinking, one including Black students and the other, White students. Findings show that PSTs evaluated Black students' thinking less favorably compared with White students. Explicit, but not implicit, attitudes, as well as reported time spent in African American communities, were factors in how PSTs rated the quality of students' mathematical thinking by race.