This research investigates whether teachers' unions influence student academic achievement in Southern states that prohibit collective bargaining. Our data are from the School and Staffing Survey and the Stanford Education Data Archive. We measure the strength of teachers' unions by union membership rate and meet-and-confer status of a district and employ a multilevel mixed-effects model to control for unobservable common characteristics of districts within each state. We find that teachers' unions have a significantly positive association with student test scores, in both math and English, particularly for Hispanic and Black students. This positive link of unions with math is concentrated on the low- and mid-socioeconomic status (SES) districts but is detected across all SES for English.