A control system for an internal combustion engine, which is capable of directly and properly calculating the most fuel-efficient torque according to operating conditions of the engine without setting or learning in advance operating lines indicative of the most fuel-efficient torques, thereby making it possible to reduce costs and enhance fuel economy. In the control system, when the engine is operated at a predetermined reference rotational speed, a plurality of fuel consumption ratio parameters associated with a plurality of estimated torques are calculated based on a provisional intake air amount-estimated torque relationship which is the relationship between provisional intake air amounts and estimated torques to be obtained when the provisional intake air amounts of intake air are supplied. Further, an estimated torque associated with a minimum value of the fuel consumption ratio parameters is calculated as the most fuel-efficient torque at the reference rotational speed.