In this study, a two-stroke outboard engine was modified to accommodate two direct fuel injectors for reactivity-controlled compression ignition combustion in one cylinder, while the production direct-injection spark ignition combustion system was maintained in the other cylinder. This setup enabled direct comparisons of the performances at equivalent operating conditions. The engine’s octane requirement for homogeneous charge compression ignition combustion was studied with primary reference fuels for ranges of engine speed, load, and delivery ratio. The resulting primary reference fuel requirement was used to determine the baseline ratio of low-reactivity fuel to high-reactivity fuel. Experiments using gasoline and diesel fuel were unsuccessful as they resulted in unstable combustion and rapid accumulation of particulate matter in the emissions-sampling equipment. Reactivity-controlled compression ignition experiments with gasoline and n-heptane (higher volatility) proved to be successful. The low-reactivity fuel fraction and the high-reactivity start-of-injection timing were found to be independent combustion-control levers. At 1500 r/min, an indicated mean effective pressure of 2.5 bar and nitrogen oxides emissions of 1.25 g/kW h, reactivity-controlled compression ignition resulted in a lower coefficient of variation in the indicated mean effective pressure, lower carbon monoxide emissions and a significantly higher gross indicated efficiency than those for the direct-injection spark ignition homogeneous mode and the direct-injection spark ignition stratified mode (36.3% versus 27.0% and 25.7% respectively); at 1200 r/min, an indicated mean effective pressure of 2.0 bar and hydrocarbon + nitrogen oxide emissions of about 16.5 g/kW h, the reactivity-controlled compression ignition efficiency was still significantly better than those for the direct-injection spark ignition homogeneous mode and the direct-injection spark ignition stratified mode (32.9% versus 25.2% and 26.6% respectively). Overall, the viability of reactivity-controlled compression ignition combustion in a two-stroke engine was demonstrated; with further design optimization, it will probably be possible to use a standard diesel fuel instead of n-heptane.