Urban heat islands, which raise environmental sustainability and public health problems in metropolitan areas, motivated the development of HeatNet, an IoT-based integrated urban environment monitoring system presented in this article. The purpose of HeatNet is to offer a low-cost and sensor-based platform to measure and visualize real-time local urban heat stress to enable targeted cooling interventions. The system consists of sensors for measuring temperature and humidity, ambient light, ultraviolet radiation and soil moisture, allowing the measurement of the main microclimate parameters in the analyzed urban area. The sensed data is forwarded wirelessly from the sensor node by the bluetooth module towards the visualization platform. The information of all sensors is then fed on by geospatial mapping methods to outline areas of heat intensity. This may be significant in policy making in applying focused cooling interventions such as afforestation or reflective roofing as a way of enhancing urban sustainability and health of the population, especially amidst the climate change.