This quantitative study investigated the relationship between ESOL co-teacher content certification and student achievement in secondary classrooms. The sample consisted of English Learners (ELs; N = 185) in a suburban district in Georgia. The study focused on three core content subjects: Algebra, Biology, and Economics. Content certification was verified through a state agency, and student achievement was assessed using Georgia Milestones test scores. A range of statistical tests, including descriptive and multilevel regression analyses, were employed to analyze the data. The results indicated that Milestones grade conversion scores were higher when teachers had dual certification (M = 65.63, SD = 10.7) versus single certification (M = 62.38, SD = 8.03). Further, multilevel regression analyses accounting for the nested structure of the data showed that teacher certification was a significant predictor of higher student scores, with dual certification predicting higher scores. This relationship did not show significant variations at the teacher level or by subject area. These findings suggest revising ESOL co-teacher scheduling in the studied context to promote equitable education for ELs.