Assessment centers (ACs) are popular selection devices in which assessees are assessed on several dimensions during different exercises. Surveys indicate that ACs vary with regard to the transparency of the targeted dimensions and that the number of transparent ACs has increased during recent years. Furthermore, research on this design feature has put conceptual arguments forward regarding the effects of transparency on criterion-related validity, impression management, and fairness perceptions. This study is the first to examine these effects using supervisor-rated job performance data as the criterion. We conducted simulated ACs with transparency as a between-subjects factor. The sample consisted of part-time employed participants who would soon be applying for a new job. In line with our hypothesis, results showed that ratings from an AC with nontransparent dimensions were more criterion valid than ratings from an AC with transparent dimensions. Concerning impression management, our results supported the hypothesis that transparency moderates the relationship between self-promotion and job performance, such that self-promotion in the nontransparent AC was more positively related to job performance than self-promotion in the transparent AC. The data lent no support for the hypothesis that participants’ perceptions of their opportunity to perform are higher in the transparent AC.