Climate change is reshaping the global water cycle, causing substantial alterations in precipitation, evaporation, and runoff patterns. These shifts are driving rapid changes in terrestrial water storage (TWS), which includes groundwater, soil moisture, surface water, snow, and ice. Declining TWS threatens freshwater security, increases the vulnerability of ecosystems and communities, and directly impacts sustainable water management—key concerns addressed under SDG 6. In parallel, intensifying climate-driven water losses align with the global challenges highlighted in SDG 13, particularly regarding adaptation and resilience. This study examines long-term TWS variations in the Susurluk Basin of Türkiye’s Marmara Region using NASA’s GRACE and GRACE-FO satellite missions. By measuring gravity anomalies caused by mass changes, GRACE enables the detection of large-scale water storage shifts. Monthly data from 41 GRACE grid points (2002–2022) were processed using the Mann-Kendall trend test at a 5% significance level. Consistent acceptance of the H1 hypothesis and universally negative Z values confirm a statistically significant and persistent decline in TWS across the basin. Results show that water storage loss accelerated dramatically between 2012 and 2022 compared to 2002–2011. The basin exhibits an overall decreasing coefficient of –0.0561, while sub-basin analyses indicate 20-year average losses ranging from –1.3 cm to –0.1 cm. These findings demonstrate a clear, worsening depletion of water resources, emphasizing the urgent need for climate-adaptive water management. The documented TWS decline underscores the relevance of this work to SDG 6 by highlighting risks to water availability and to SDG 13 through evidence of climate-induced hydrological change.