Despite the importance of surface soil moisture for monitoring moisture conditions and supporting water resource and agricultural management, regional information on its spatiotemporal variability and relationships with precipitation and vegetation moisture in Gilan Province remains limited. This study aimed to investigate the temporal and spatial patterns of surface soil moisture and assess its relationships with annual precipitation and the Normalized Difference Moisture Index (NDMI). SMAP L4 soil moisture data for 2015–2020 were extracted and processed using the Google Earth Engine (GEE) platform. NDMI was derived from satellite imagery, while annual precipitation data were obtained from meteorological stations across the province and compared with soil moisture variations. The results revealed a distinct seasonal pattern, with mean soil moisture reaching approximately 25–30% during wet months and declining to below 10% during dry months. A relative decline in seasonal soil moisture peaks was observed from 2018 onward, while soil moisture values ranged from 4.9% to 27.9% during the study period. Annual variations in NDMI and soil moisture were consistent in some years but not throughout the entire study period. Linear regression analysis indicated a strong positive relationship between annual mean soil moisture and precipitation (R² = 0.86). Overall, SMAP data provided useful information for monitoring seasonal and interannual soil moisture variations in Gilan Province, while NDMI can serve as a complementary indicator of vegetation moisture conditions. The integrated use of soil moisture, precipitation, and remote-sensing indices can support drought monitoring and regional water and agricultural management.