This research investigates the fire resistance of Autoclaved Aerated Concrete (AAC) blocks, a lightweight building material gaining popularity. Despite its widespread use in the Sri Lankan construction industry, very few studies have been carried out on the fire endurance of AAC material, a crucial factor in construction safety. Data collection encompassed visual inspection and quantitative analysis, ensuring a robust dataset for comprehensive examination. The study primarily focused on investigating the residual mechanical properties of AAC blocks when subjected to various temperature regimes (up to 1000 °C) and exposure durations (up to 90 min). These conditions were applied using an electrical furnace, and the mechanical properties were evaluated using compressive strength and splitting tensile strength tests. The thermal conductivity assessment indicated that AAC blocks maintain strong insulation properties up to 300 °C. Significant structural cracks emerged at 950 °C, severely impacting thermal conductivity. The residual compressive strength analysis showed a consistent decrease with rising temperatures, with strength becoming negligible at 900 °C. The minimum required strength can be maintained up to 500 °C, with a 16% reduction in compressive strength compared to the initial value. Splitting tensile strength tests revealed a similar trend, with residual tensile strength diminishing as temperatures increased. Specifically, the splitting tensile strength decreased by 55% as the temperature rose from room temperature to 700 °C. Hence, the findings denote AAC blocks maintain structural integrity up to 500 °C. However, their effectiveness diminishes beyond this temperature due to significant thermal degradation.