This rainfall monitoring system uses a rainfall sensor made from flow meter YF-S201, using microcontroller Wemos D1 which is based on ESP8266. The purpose of this study of the rainfall measurement system is to create a real-time rainfall information system that can be used locally in the wanted area, and by using the Internet of Things (IoT) so that it can monitor the weather from a distance and real-time by displaying rainfall data through Blynk application on the smartphone. This study began with designing, manufacturing, and programming system. Then testing rainfall sensor. The results of the test are the relative error value of the rainfall sensor is 0.31 mm to 20.16 mm and the RMSE value is 0.59.