This study aims to design and implement an Internet of Things (IoT)-based monitoring prototype of a water treatment plant to improve the quality of brackish water in Pontang Legon Village, Serang Regency. The developed system integrates an electrocoagulation process with a multilayer filtration unit consisting of a sediment filter, magnesium (Mg) cartridge filter, cation resin filter, and chlorine taste and odor filter. The monitoring system employs an ESP32 microcontroller connected to a DS18B20 temperature sensor, a PH-4502C pH sensor, and a Total Dissolved Solids (TDS) sensor to enable real-time water quality monitoring. Measurement data are displayed on an LCD and transmitted to the ThingSpeak platform, which can be accessed through the ThingView application. The research method follows a Research and Development (R&D) approach using the Plomp model. Experimental results indicate that optimal operating conditions were achieved at a voltage of 12 V with a processing time of 120 minutes, resulting in a pH increase of 128.15% and a TDS reduction of 11.51%, while maintaining water temperature stability within the range of 27–31 °C. These results demonstrate that the proposed system has significant potential as a village-scale clean water treatment solution for coastal areas.