Liquefied Natural Gas (LNG) has recently become a popular fuel in the shipping industry due to its low emissions and high energy density. However, it requires one or more bulky cryogenic tanks for storage onboard. Considering a traditional fast ferry, it is hard to find onboard a location for tanks, thus hindering the retrofit of existing ships. In this context, Compressed Natural Gas (CNG) might be a viable solution, since it can be more flexibly stored onboard and has greater availability in minor ports compared to LNG. Hence, this article investigates the feasibility of retrofitting the propulsion system of a fast ferry to employ CNG as fuel. After a review of the pros and cons of CNG and LNG as alternative fuels to marine diesel oil (MDO), a critical analysis of the technical requirements for retrofitting a fast ferry with CNG propulsion systems is carried out. Defined the layout and changes of the refitted unit, its performances are assessed on a test operative scenario. The study concludes that CNG retrofitting is technically feasible and provides several benefits, including lower emissions, higher levels of performance and higher reliability compared to LNG. Nevertheless, the retrofit requires significant changes to the ship layout and its fuel system to fit the required number of CNG cylinders.