This article presents a meta-analysis of 40 studies conducted during the first two decades of the 21st Century that have analyzed the relationship between teacher characteristics and competencies and the academic performance of secondary school students, which contributed a total of 202 effects. The effect sizes were estimated using Fisher's transformation and a random-effects model was proposed to study the summary effect. The results show that teachers’ characteristics and competencies explain 9.2% of the differences in students’ performance. Although the overall effect can be considered as moderate, a closer examination of specific teacher characteristics and competencies revealed some larger effect sizes. This is the case of teachers’ reflective attitude, their professional development, and teaching self-efficacy.