This laboratory-scale study examined how soil particle-size distribution (PSD), especially the median diameter d₅₀, affects slope runoff and sediment yield, and evaluated d₅₀ as a simplified indicator for erosion prediction. Simulated-rainfall experiments were performed using a rainfall simulator and an adjustable steel flume. Five soils with contrasting PSDs (sandy loam to silty clay; d₅₀ = 0.00524–0.06105 mm) were tested under three rainfall intensities (60, 90, and 120 mm h⁻¹) for 45 min events, with two replicates per treatment (30 events total). Runoff rates increased with clay content: sandy loam < mixed sandy loam < loam < mixed silty clay < silty clay. Partial correlation analysis showed strong associations between runoff rates and d₁₀, d₃₀, d₅₀, d₆₀, and C_c, with a partial correlation coefficient of −0.78 for d₅₀. Sediment yield rates showed the opposite trend, decreasing as clay content increased. Cumulative sediment yield increased with d₅₀ via a positive power-law relationship, with calibration R² values of 0.91–0.95 across rainfall intensities. The proposed d₅₀-based erosion model achieved a calibration R² of 0.97 with two fitted parameters (k₁ and b) and inputs of rainfall erosivity, slope length, and slope steepness. These laboratory-scale results provide a preliminary basis for incorporating d₅₀ into erosion-prediction models for production and construction projects; however, field validation is required before engineering application.