This study explores the incorporation of sage (Salvia officinalis) as a bio-based filler in flexible polyurethane foams (FPUFs). Sage was added to FPUFs in varying concentrations (5, 10, and 15 wt.%) to examine its influence on the foaming process, microstructure, density, water absorption, and compression behavior. Results revealed that sage delayed the foaming reaction, reduced exothermic reaction temperatures, and enhanced foam expansion. Microstructural analysis showed finer cell structures and denser foam matrices with sage incorporation. Foam filled with 5 wt.% of sage exhibited reduced compression resistance; however, increasing the filler concentration led to improvements in both compression resistance and density. Sage also increased water absorption due to its hygroscopic nature, though this effect plateaued at higher concentrations. These findings demonstrate the potential of sage as a sustainable additive to produce high-performance FPUFs suitable for various industrial applications.