This study evaluates the performance of lightweight mortar incorporating recycled expanded polystyrene (EPS) as a partial replacement for natural river sand. Targeting non-structural applications plastering, rendering, and masonry bedding a 25% volumetric sand replacement was investigated. The water-to-cement ratio was maintained between 0.45 and 0.50 using an OPC:sand ratio of 1:3 by volume. Results show a 28-day compressive strength of 9.375 MPa (exceeding the 6 MPa threshold for non-structural use), a thermal conductivity of 1.40 W/m·K, and a dry density of approximately 1200–1500 kg/m³. Water absorption increased modestly and remained acceptable for interior applications. The findings confirm that 25% EPS substitution represents an optimal balance between mechanical adequacy, weight reduction, and thermal performance for sustainable construction in developing economies