We present a prototype of a real‐time system for automatic aftershock forecasting in Japan. Using real‐time seismicity data from the High Sensitivity Seismograph Network of the National Research Institute for Earth Science and Disaster Resilience (NIED), the system automatically generates rapid aftershock forecasts. The system starts to issue a time‐dependent forecast about 3 hrs after a mainshock and keeps updating it hourly. The forecast includes the estimated occurrence probabilities of earthquakes with magnitudes equal to or greater than a mainshock. Because an official procedure for aftershock forecasting in Japan does not provide aftershock probabilities in the first week after a large earthquake, this rapid response of the system after a mainshock can complement the existing procedure. We demonstrate the system’s performance for the cases of three inland earthquakes that occurred after the establishment of the system in April 2017. We also discuss the potential usefulness of our procedure for forecasting large earthquakes by conducting a retrospective forecast test for the 2016 Kumamoto foreshock–mainshock–aftershock sequence based on the real‐time seismicity data. Our monitoring system is useful for providing continuous timely information on time‐dependent aftershock probabilities.