The study experimentally evaluated the environmental and operational performance of a combined exhaust gas recirculation–diesel particulate filter (EGR–DPF) retrofit system installed on a Yanmar S185L-ST medium-speed marine diesel engine under representative marine operating conditions. The retrofit configuration was designed to simultaneously reduce nitrogen oxides (NOx) and particulate matter (PM) emissions while maintaining acceptable engine performance and operational stability. Experiments were conducted under different engine speed and load conditions along the propeller characteristic curve. The results showed that the EGR system effectively reduced NOx emissions under all investigated operating conditions, with reduction efficiencies ranging from 4.28% to 11.69%. The highest NOx reduction was observed at 79% rated speed and 50% engine load. The DPF system demonstrated strong PM filtration capability, with PM reduction efficiencies ranging from 28.13% to 97.67%, particularly under medium and high engine speed conditions. Evaluation of brake-specific fuel consumption (BSFC) indicated that the retrofit system did not significantly deteriorate engine performance, and stable engine operation was maintained throughout the experiments. The results confirm that the proposed EGR–DPF retrofit configuration provides a practical, low-cost, and environmentally effective solution for reducing emissions from existing marine diesel engines, particularly for inland and coastal vessels operating in developing countries.