An air/fuel ratio control system is provided for an internal combustion engine having a plurality of cylinders and an air/fuel ratio sensor. The air/fuel ratio sensor is disposed at an exhaust gas merging portion of an exhaust passage where exhaust gas discharged from the plurality of cylinders merges together. The system includes an air/fuel ratio control device, an abnormality diagnosis device, and an enabling device. The air/fuel ratio control device individually controls an air/fuel ratio of each of the plurality of cylinders based on an output of the air/fuel ratio sensor. The abnormality diagnosis device determines whether abnormality of the air/fuel ratio sensor exists. The enabling device enables the air/fuel ratio control device to execute the controlling of the air/fuel ratio of each of the plurality of cylinders when the abnormality diagnosis device determines that the abnormality of the air/fuel ratio sensor does not exist after starting of the engine.