The objective of the present study is to synthesize semi-rigid polyurethane foam/expanded perlite (SrPUF/EP) composites using different amounts of EP, a natural and environmentally friendly alternative to traditional halogen-containing flame-retardants in a one-step process. This study investigates the mechanical, physical, thermal, and morphological properties, as well as the flame-retardant and smoke release behaviors of composites. The cone calorimetry test results indicated that the amounts of carbon monoxide and carbon dioxide released during combustion process decreased by 43% and 37%, respectively, in the 8% EP-added SrPUF/EP composite (4SrPUF). The amount of char residue formed after complete combustion of 4SrPUF composite was found to be 2.7 times higher than that of pure SrPUF. In the UL-94 test, it was observed that when 4% or more EP was added to SrPUF, the liquid dripping during combustion ceased entirely, and after complete combustion, the main structure remained in the form of char without disintegration. The flexural strength of 1SrPUF, 2SrPUF, and 3SrPUF exhibited increases of 62%, 69%, and 121%, respectively, in comparison to SrPUF (47.15 kPa). The results of this study indicate that EP has the potential to serve as an environmentally friendly and cost-effective flame-retardant, thereby enhancing the fire resistance properties of SrPUF.