Abstarct: This ex post facto study investigated the effect of teacher certification type on Iraqi sixth preparatory students' English proficiency, as measured by the Ministerial English Examination. The study was guided by three research questions examining the main effect of certification type and its interactions with teacher experience and school type. The theoretical frxamework integrated Shulman's Pedagogical Content Knowledge (PCK), Stronge's Teacher Effectiveness Theory, Human Capital Theory, and Darling-Hammond's Teacher Quality frxamework. The study employed a quantitative ex post facto research design with a two-stage cluster sampling procedure. Five secondary schools were randomly selected from Baghdad, and from each school, one class taught by an internationally certified teacher and one class taught by a domestically certified teacher were randomly selected. The final sample consisted of 210 sixth preparatory students (105 per certification group). Data were collected from archival school records of Ministerial English Examination scores and certification verification documents. Data were analyzed using descxriptive statistics, assumption testing, and one-way and factorial ANCOVA. The findings revealed that teacher certification type had a statistically significant effect on student English proficiency, F(1, 206) = 42.37, p < .001, η²p = .17, with students taught by internationally certified teachers outperforming those taught by domestically certified teachers by 7.40 adjusted mean points. A significant interaction was found between certification type and teacher experience, F(1, 205) = 16.55, p < .001, η²p = .07, indicating that the advantage of international certification was more pronounced among less experienced teachers. A significant interaction was also found between certification type and school type, F(1, 205) = 11.75, p < .001, η²p = .05, indicating that the advantage of international certification was more pronounced in public schools. All three null hypotheses were rejected. The findings contribute to the theoretical understanding of teacher certification effectiveness in developing country contexts and have practical implications for teacher certification policy, teacher preparation programs, and school administration in Iraq. The study recommends investments in international certification for novice teachers and targeted placement of internationally certified teachers in public schools.