Transformers are essential components in power systems because they regulate voltage levels for energy distribution. One of the most common issues is excessive temperature rise (overheating), which can reduce insulation life, lower reliability, and even cause severe damage or fire. Conventional temperature monitoring systems generally rely on manual inspections, making them less effective in detecting temperature changes in real time. This study aims to design a prototype of a transformer temperature monitoring system based on the Internet of Things (IoT) using the DS18B20 temperature sensor as the detector, NodeMCU ESP8266 microcontroller as the data processor, 20x4 I2C LCD for local display, and a Buzzer as an alarm indicator. The temperature data are transmitted via Wi-Fi and displayed on the Blynk application for real-time remote monitoring. The research method includes hardware and software design, component testing, and field data analysis. The test results show that the system successfully displays transformer temperature in real time with stable and reliable performance. During seven days of testing, the average transformer temperature ranged between 34.60°C – 38.49°C, with the minimum recorded at 33.95°C and the maximum at 43.00°C. The accuracy level of the DS18B20 sensor in this system reached above 98%, proving that the prototype provides consistent and dependable measurement. In addition, the Buzzer automatically activates when the temperature exceeds 70°C. Therefore, the developed prototype offers a practical, accurate, and efficient solution for transformer temperature monitoring and helps prevent the risk of overheating.