Floods are ranked first as natural disasters in Indonesia and are the most frequent in most of the Central Java Region (BNPB, 2020). There are 10 (ten) sub-districts in Semarang City that are potential flood-prone (BPBD Semarang City, 2021). Objective: to make a prototype of a flood monitoring system based on applications, sirens and warning notifications for flood-prone areas. Where, this flood monitoring system can provide water level data quickly, accurately, easily accessible anywhere and anytime. Research Methods: The methods used are problem identification, literature study, design and planning, testing and analysis, data collect, and report writing. Result and Conclusions: 1. The Flood Detector Based On Application (FODBOA) can provide water level data quickly, accurately and easily accessible to the public because it uses an android-based cellular phone as an information medium for the flood monitoring system. 2. Warning notifications and sirens are issued when the water level reaches dangerous. 3. Monitoring data in the application can be read in a fast time, which is an average of 1,003 seconds every 1 (one) time of delivery.