This study evaluated the bio-stimulatory effect of yeast foliar application (0, 10, and 20 g·L⁻¹) on true potato seed (TPS) and berry productivity in two potato cultivars (Naima and Eclat) under Mediterranean climatic conditions (400 mm annual rainfall; 15.5°C mean temperature) in Ram Hamdan, Syria (2023–2024). Using a CRD design with five replicates, results revealed significant differences (p < 0.05): Eclat outperformed Naima in floral density (62.9 vs. 41.16 flowers·plant⁻¹ at 10 g·L⁻¹), berry count (21.8 berry·plant⁻¹), and TPS yield (1.94 g·plant⁻¹), achieving the highest area-based productivity (77.7 kg·ha⁻¹). Conversely, Naima exhibited superior berry weight (5.26 g·berry⁻¹ at 20 g·L⁻¹), reflecting enhanced dry matter partitioning. Yeast application also improved vegetative growth (3.6 stems·plant⁻¹; 182.2 cm² leaf area·plant⁻¹ for Naima at 10 g·L⁻¹) and accelerated flowering (43.4 vs. 49.8 days in control). Seed physiological quality peaked at 10 g·L⁻¹ (65.2% germination for Eclat). The interaction analysis highlighted cultivar-specific optima: 10 g·L⁻¹ for Eclat vs. 20 g·L⁻¹ for Naima. These findings advocate yeast as a viable bio-stimulant for TPS production, particularly in high-yielding cultivars like Eclat, with tailored concentration recommendations (10 g·L⁻¹ for Eclat; 20 g·L⁻¹ for Naima) under Mediterranean conditions.