Fibre-reinforced polymers (FRP) have gained significant traction in construction, especially for bridge applications, due to advantages like reducing self-weight by 30–60% and construction time by up to 80%. However, bridges experience variable thermal conditions (daily, seasonal, and annual) that impact their static and dynamic responses, leading to thermal stresses from non-uniform temperature distributions. These temperature variations can significantly affect structural behaviour, given that the mechanical properties of polymer composites are highly temperature-dependent. This means that even minor temperature changes can significantly alter them, especially when approaching the glass transition temperature. FRP-based bridge structures typically require a top layer, usually concrete, whose thermal properties must be considered in thermal analyses. Mitigation methods should be explored to minimize temperature increases caused by this layer. This study investigated the thermal behaviour of a hybrid FRP sandwich panel with a glass-FRP box structure, a polyurethane foam core, and a top layer of steel fibre reinforced self-compacting concrete (SFRSCC). Outdoor tests in Guimarães, Portugal, analysed the effect of the top layer on temperature distribution. Six PT100 temperature sensors, positioned at the mid-span central axis of the panels, monitored temperature distributions under different conditions: (i) with the top layer of SFRSCC in its natural colour vs. disregarding the top layer (direct exposure of the GFRP) and (ii) with the top layer painted in a brighter vs. painted in a darker colour. The findings underscore the importance of considering the thermal properties of the top layer in designing FRP-based bridge structures to ensure their performance and longevity.