This paper presents the development of a computational model for large-scale oil spill fires on water in tsunamis. A tsunami following an earthquake with the moment magnitude of 9.0 off the coast of Japan on March 11, 2011, which is often referred to in Japan as the 2011 Great East Japan Earthquake and Tsunami, caused large-scale oil spill fires spreading over Kesennuma Bay in Miyagi Prefecture. A large quantity of marine diesel oil were spilled from fuel tanks destroyed by the tsunami, and contributed to the fires by igniting on the sea. Although the fires on the sea caused a number of secondary fires by spreading to buildings, ships, and forests, safety measures against such tsunami-induced oil spill fires have not been sufficiently considered in Japan because of the lack of numerical simulation methods for the fires. Here, assuming that the tsunami is simulated using a proven numerical code that is capable of predicting tsunami propagation and inundation based on a theory for long waves, we newly modeled the spread and burning behavior of oil on water in tsunamis, and integrated the models for tsunami flow, oil spread and oil burning. In order to model the spread and burning behavior of oil in tsunamis, we assumed that the oil spill fires on water in tsunamis are the assembly of burning oil particles floating on water. Therefore, we formulated the motion of the oil particle in tsunamis, the fire spread between oil particles, and the heat release of the oil particle due to combustion. Moreover, we numerically simulated the oil spill fires at Kesennuma Bay in the 2011 Great East Japan Earthquake and Tsunami in order to validate the proposed model. From the calculated results, we concluded that (1) the dynamic states of oil spill fires at Kesennuma Bay had depended largely on the tsunami flow, and (2) the proposed model could reasonably explain the survey report by the fire departments on the dynamic states of oil spill fires and the burned-out areas on the ground.