Students’ educational outcomes are predicted by their noncognitive characteristics, including Big Five personality domains. Although theories of teaching and learning suggest that teacher noncognitive characteristics also impact student outcomes, such characteristics are rarely studied systematically. We propose that the Big Five personality domains of teachers are associated with teacher effectiveness. Furthermore, we test two potential moderators of these relationships: (1) source of teacher personality report (student reports may show stronger effects than teacher self-reports) and (2) frame of reference (contextualized “at school” personality items for teacher self-reports may show stronger effects than noncontextualized standard personality items). Multilevel regressions were conducted on the data collected from secondary school students (N = 2,082) and their mathematics and English teachers (N = 75). We statistically controlled for student and teacher gender, student previous academic achievement, and student personality. Teacher personality predicted the subjective measures of teacher effectiveness—the strongest predictors were conscientiousness for teacher academic support, agreeableness for teacher personal support, and neuroticism for student performance self-efficacy. Teacher personality did not predict the objective measure (student academic achievement). These effects were moderated by source of personality report but not by frame of reference. The possibility of including personality as part of the initial teacher trainee selection procedure in the future is briefly discussed.