Extreme heat events represent an escalating risk for public health in Southern Europe, with Greece being particularly vulnerable. In the framework of the HEATMETEO-GR project, funded by the European Climate Foundation (ECF), a biometeorological early warning system for extreme heat was operationally implemented at the national scale for summer 2025. The system integrates high-resolution meteorological forecasts with human-biometeorological indices (Modified Physiologically Equivalent Temperature, mPET) to provide impact-based heat-health warnings, disseminated via www.meteo.gr. This paper presents the design, operational chain, and examples from its implementation, highlighting representative heat episodes of summer 2025 and discussing lessons learned for future improvements.