© IMechE 2018.Energy-saving, stricter regulations on pollution prevention, increasing energy efficiency and reducing operational costs have become common and primary issues for maritime industry, recently. One of the methods to improve these requirements onboard is to use a waste heat recovery system based on organic Rankine cycle. In this article, organic Rankine cycle–based waste heat recovery system for a chemical/oil tanker is investigated at different engine operating conditions by thermodynamic, environmental and economic analyses. The jacket cooling water, scavenge air, exhaust gas and combination of these three sources are used as the waste heat sources. R245fa is selected as the working fluid. The performance parameters of four different organic Rankine cycle–based waste heat recovery systems integrated with the main engine of the tanker are calculated and presented. The results show that by using the organic Rankine cycle–based waste heat recovery system onboard, it is possible to increase the overall thermal efficiency of the ship’s power plant by more than 6% and the combined organic Rankine cycle–based waste heat recovery system can meet all navigation electricity demand when the engine is operated at 82% maximum continuous rating or higher engine loads. In comparison with other organic Rankine cycle–based waste heat recovery systems, the combined organic Rankine cycle–based waste heat recovery system has the highest capital cost, but it has the shortest payback time. Furthermore, this system can reduce the ship emissions by about 6.9%.