More judicious application of phosphorus (P) fertilizer can moderate the impacts of global warming on crop water productivity (WP) in (semi-)arid regions, but its long-term feedback on future climate and maize ecoregions remains unclear. This study quantified future changes in maize WP under multiple climate scenarios in the Yellow River Basin (YRB) of China and evaluated the potential of optimized P fertilization to mitigate WP decline. Based on the process-maize-model and combined with 5 global climate models (GCMs) & times; 3 shared socioeconomic pathway scenarios (SSPs; SSP126, SSP370, and SSP585), our results showed that WP of maize will decrease by an average of 5% in future relative to the baseline period in the whole YRB, with a greater decline under the SSP585 scenario. WP in different ecoregions also declined, particularly in the Huang-Huai region (II), which experienced significant decreases (p < 0.05) under SSP585 in the 2021-2040 (2030s), 2041-2070 (2050s), and 2071-2100 (2080s). Rising temperatures were the primary driver, with a threshold effect observed: WP turned point at above 20 degrees C in eastern YRB (regions I and II), while in the rest of other ecoregions (III-V), it peaked around 12 degrees C then declining. After crossing temperature thresholds under an ensemble of projected climate scenarios, optimizing P fertilization increased maize WP by an average of 10.5% compared with the no-fertilization treatment. A combined nitrogen (N) and P application (180 kg ha(-1) and 90 kg ha(-1) respectively) maximized WP across all SSPs. These findings provide critical temperature thresholds and effective fertilization strategies for sustaining future maize production and water consumption in the YRB and similar regions.