International differences in teacher quality are commonly hypothesized to be a key determinant of the large international student performance gaps, but lack of consistent quality measures has precluded testing this. Using unique assessment data, we construct country-level measures of teacher cognitive skills. We find substantial differences in teacher cognitive skills across countries, and these are strongly related to student performance. Results are supported by fixed-effects estimation exploiting within-country between-subject variation in teacher skills. Observed country variations in teacher cognitive skills are significantly related to differences in women’s access to high-skilled occupations outside teaching and to salary premiums for teachers.