This article examines the association between the equity concentration and agency costs as well as the impact of agency costs on performance of non-financial firms listed on the Saudi Stock Exchange (Tadawul). These relations are examined by using dynamic panel data and a two-step robust system estimation for the period 2010–2013. The study uses three proxy variables to measure agency costs. The results show that the equity concentration has no significant impact on agency costs, and the agency costs have no significant impact on firms’ performance. In addition, the study shows no evidence to support the agency theory in non-financial firms listed on the Saudi Stock Exchange (Tadawul). This study provides a better understanding of the association between equity concentration, agency costs and a firm’s performance.