Optimizing water productivity and nutrient management under rainfed conditions has become critical for sustainable sunflower production due to increasing climate variability and water scarcity in semi-arid regions. This study was conducted to investigate the relationships between water productivity (WP), evapotranspiration (ET), sunflower yield, plant nitrogen (N) content, oil content, and selected soil chemical properties under rainfed conditions in the Thrace Region of Türkiye. Field experiments were carried out at different locations between 2013 and 2016 using the sunflower cultivar ‘Tunca’. Five N fertilizer levels (0, 30, 60, 90, and 120 kg N ha⁻¹) were applied under rainfed conditions in a Latin square experimental design with five replications. Seasonal crop evapotranspiration was calculated using the soil water balance method, while WP was determined as the ratio of grain yield to evapotranspiration. Regression and Pearson correlation analyses were performed to evaluate the relationships among the investigated parameters. Seasonal ET values varied between 198 and 464 mm depending on year and location. Strong positive linear relationships were determined between WP and sunflower yield in all experimental sites, indicating that improved WP significantly increased grain yield. Similarly, positive relationships were observed between WP and plant N content, suggesting that efficient water use enhanced N uptake and utilization. In contrast, WP generally showed negative relationships with sunflower oil content, indicating that increasing yield and N accumulation may reduce oil concentration in seeds. Correlation analyses demonstrated that ET was closely associated with soil chemical properties. Potassium (K₂O) generally exhibited positive relationships with ET, whereas high CaCO₃ content, elevated pH, and micronutrient imbalances negatively affected plant water consumption. The results indicated that both soil fertility and water availability play critical roles in regulating evapotranspiration, nutrient uptake, yield formation, and oil accumulation in sunflower under semi-arid conditions. Overall, the findings demonstrated that improving WP is essential for sustainable sunflower production under rainfed conditions. In addition, soil chemical properties should be carefully considered in water management and nutrient management strategies in order to optimize yield, plant nutrition, and oil quality in sunflower cultivation.