Plant-available water (PAW) is a primary regulator of plant growth, ecosystem productivity, and drought resilience, yet its spatial heterogeneity and long-term response to hydrothermal changes remain poorly quantified at continental scales. Here, we quantify historical (2005-2015) and projected (2030-2100) changes in growing-season PAW across the continental United States (CONUS) by integrating site-level soil moisture observations with environmental predictors and bias-corrected downscaled Coupled Model Intercomparison Project phase six (CMIP6) climate projections. Root-zone PAW (0-30 cm) was estimated from soil moisture content, field capacity, and wilting point, thereby capturing coupled soil and climatic controls on plant water availability. Ensemble CMIP6 projections under two Shared Socioeconomic Pathways (SSP): SSP2-4.5 and SSP5-8.5 indicate widespread declines in growing-season PAW, driven primarily by increasing temperature and  vapor-pressure deficit that increase atmospheric evaporative demand, outpacing projected precipitation gains, with substantially stronger drying under SSP5-8.5. Compared to baseline conditions, PAW declines across approximately 6.9-19.8 % of the CONUS, with the largest reductions concentrated in the Great Plains, western Corn Belt, and Intermountain West ecoregions. In contrast, 76.2-84.6 % of the region shows no significant change, while 4.0-8.5 % exhibits increases, primarily across northern forested regions. Collectively, these findings identify emerging hotspots of PAW limitation and highlight growing risks to crop production and rangeland resilience, emphasizing the need for region-specific adaptation and water-management strategies under future environmental conditions.