Energy efficient and environmentally friendly shipping have been primary concerns for the maritime industry. One of the alternatives to overcome these issues onboard is organic Rankine cycle (ORC) waste heat recovery system (WHRS). In this study, a transcritical ORC WHRS for a marine diesel engine is investigated at different engine operating loads by thermodynamic and environmental analysis. The engine exhaust gas is used as the waste heat source and R152a is selected as the working fluid. The energetic, exergetic and environmental parameters are analysed and the performance optimisation is conducted by using genetic algorithm. The results indicate that by employing the ORC system onboard, it is possible to increase the overall thermal efficiency of the ship power generation system by more than 2.5% and the system can save up to 678.1 tonnes CO(2)per year when the system is operated at the optimal conditions.