The main objective of this study was to investigate the interactions between phosphorus (P) and iron (Fe) fertilizers under different irrigation regimes and their effects on nutrient uptake, yield, and quality of sugar beet. The experiment was conducted at the Varamin Agricultural Research Station, Iran, during the 2021- 2022 growing seasons. The study used a split-plot design with three replications. The main factor consisted of three irrigation regimes (100%, 75%, and 50% of water requirement), and the sub-factor included six combinations of phosphorus (0, 75, and 150 kg/ha) and iron (0 and 5 kg/ha) fertilizers. Phosphorus use efficiency (PUE), phosphorus uptake efficiency (PUpE), and phosphorus utilization efficiency (PUtE) were measured. Results showed that severe water stress (50% of water requirement) reduced PUE by 25-30%, PUpE by 22-33%, PUtE by 26-37%, root yield by 26-35%, and white sugar yield by 26-37% compared to full irrigation. In contrast, the combined application of 150 kg/ha superphosphate with 5 kg/ha iron, depending on moisture conditions, increased PUE by 10-40%, PUpE by 14-55%, PUtE by 11-46%, root yield by 10-40%, and white sugar yield by 11-46% compared to the control (no fertilizer). This fertilizer combination also improved juice quality by decreasing sodium content by 25-32%, harmful nitrogen by 25-40%, and increasing potassium content by 21-25%, juice purity by 2.5-2.7%, and sucrose content by 0.4-1.3%. Overall, the balanced application of 150 kg/ha superphosphate with 5 kg/ha iron was the most effective fertilizer combination for improving phosphorus use efficiency, yield, and quality of sugar beet, even under water stress conditions.