A control system for an internal combustion engine, which is capable of quickly and properly ensuring excellent fuel economy of the engine even when atmospheric pressure has changed. The control system includes an ECU. The ECU performs weighted average calculation on lowland map values and highland map values, based on atmospheric pressure, to thereby calculate map values of a demanded torque that minimize a fuel consumption ratio of the engine in the atmospheric pressure, and calculates engine demanded output. The ECU calculates demanded torque and demanded engine speed using the map values and the engine demanded output, respectively. The ECU controls the engine using the demanded torque.