This study aims to study the factors that influence the Professionalism and Ethical Behavior of Auditors at the Financial and Development Supervisory Agency (BPKP) North Sumatra Province and produce a model of Mediation of Professionalism on the Influence of Moral Intensity and Religiosity on Auditor Ethical Behavior at Financial and Development Supervisory Agency (BPKP) NorthSumatra Province. The strategy employs a quantitative approach. The sample includes all Financial and Development Supervisory Agency (BPKP) North Sumatra Province internal auditors. Using a technique called "saturated sampling," criteria from all internal auditors were used as samples. One hundred forty-three internal auditors were chosen as the final sample. The method used is Partial Least Square (PLS). The results showed that moral intensity and religiosity positively affect internal auditor professionalism and ethical decision-making, and internal auditor professionalism positively affects ethical decision-making. Then, moral intensity and religiosity do not significantly influence ethical decisionmaking through internal auditor professionalism.