Potassium (K) plays a central role in potato physiology and tuber bulking, yet foliar K programs may differ markedly depending on application rate and fertilizer source. This study evaluated the effects of foliar K source and dose on vegetative growth, tuber yield, and leaf nutrient status of potato (Solanum tuberosum L.) under controlled conditions. The experiment was conducted in a growth chamber from 2 February to 2 May 2025 using certified seed tubers of cv. Çanlıbel (Azerbaijan), obtained via the apical meristem technique and pre-sprouted for 20 days at 25 ± 2°C. A clay soil was used as the growing medium, and pots were maintained at 100% plant-available water throughout the experiment. Treatments were arranged in a completely randomized design with three replications (n = 3) and included a control (no foliar K) and foliar applications supplying 500, 1000, or 2000 mg kg⁻¹ K₂O from four K sources: KCl, K₂SO₄, KNO₃, and potassium silicate (K₂O₃Si). Foliar sprays were applied twice during the tuber bulking period using Tween-20 (0.1%) as a wetting agent. Foliar K significantly improved canopy development, increasing stem number, stem diameter, plant length, and total leaf number relative to the control, with responses generally strengthening as K₂O dose increased. Tuber yield per plant increased from 531 ± 25 g in the control to 663 ± 41 g under 2000 mg kg⁻¹ K₂O as KNO₃, while tuber number varied within a comparatively narrow range, indicating that yield gains were driven primarily by improved tuber bulking rather than tuber set. Leaf macronutrient concentrations (especially N and K) and micronutrients (Fe, Cu, Zn, and Mn) were enhanced by foliar K, with the greatest and most consistent improvements observed for KNO₃, followed by potassium silicate and K₂SO₄. Overall, foliar K application during tuber bulking-particularly KNO₃ at ≥1000 mg kg⁻¹ K₂O—was effective for improving potato growth, yield, and nutritional status under non-limiting water conditions.