There is a close relationship between potassium (K) and nitrogen (N). However, the roles of K under high N conditions remain unclear. Using a hydroponics approach, we monitored the morphological, physiological, and molecular changes in M9T337 apple (Malus domestica) rootstocks under different nitrate (10 and 30 mmol·L⁻¹) and K supply (0.5, 6, 10, and 20 mmol·L⁻¹ K⁺) conditions. Results revealed that high nitrate inhibited the root growth of M9T337 rootstocks, downregulated the expressions of K transporter genes (MdPT5, MdHKT1, and MdATK1), and reduced the net and K⁺ influx at the surface of roots, thereby resulting in an N/K imbalance in rootstocks. Further investigation showed that 10 mmol·L⁻¹ K increased the activity of N metabolic enzymes (NR, GS, NiR, and GOGAT), upregulated the expressions of genes related to nitrate uptake and transport (MdNRT1.1, MdNRT1.2, MdNRT1.5, and MdNRT2.4), promoted ¹⁵N transport from the roots to the shoots, optimized leaf N distribution, and improved photosynthetic N utilization efficiency under high nitrate conditions. These results suggest that the negative effects of high nitrate may be related to the N/K imbalance and that reducing N/K in plants by increasing K supply level can effectively alleviate the inhibition of N assimilation by high nitrate stress.