This study examined the relationship of personality traits with academic achievement, while controlling for cognitive ability. We considered two domains (Mathematics and English as a foreign language) and three achievement measures, capitalizing on a sample of N = 3637 students in upper secondary education (year 13, age M = 19.92 years) in Germany. First, we aimed to replicate previous results on grades and test scores. Second, we aimed to extend the body of research by adding final examinations—a school-based performance test—as a third measure. Our findings indicate an incremental predictive validity of personality traits for domain-specific academic achievement beyond cognitive ability. Conscientiousness predicted grades and final exams in both domains. Results for test scores were domain-specific: conscientiousness predicted mathematics test score, whereas openness was associated with English test score. Relationships with personality traits varied depending on the domain, the measure used, and the covariates included in the model.