Soil moisture regulates infiltration, evapotranspiration, runoff, vegetation dynamics and drought response, but its spatial and seasonal variability is difficult to characterise from conventional observations alone. This study evaluates the seasonal predictability of soil moisture across Bihar, India, using multi-source Earth observation and environmental variables within a machine-learning framework. A final analytical dataset comprising 12,897 observations from 1,279 sampling locations was developed for June 2025–May 2026. The predictor set included latitude, longitude, elevation, slope, aspect, land use/land cover, clay, sand, silt, Normalized Difference Vegetation Index (NDVI), Normalized Difference Water Index (NDWI), land surface temperature (LST), and rainfall. Monthly mean ERA5-Land volumetric soil water content for layer 1 was used as the target variable. Separate Random Forest (RF) models were developed for the Monsoon, Post-Monsoon, Winter and Pre-Monsoon seasons. To minimise spatial information leakage caused by repeated observations at the same locations, point-aware train–test validation was applied. RF performance varied substantially among seasons, with R² values of 0.8984, 0.9693, 0.6681 and 0.9773 for Monsoon, Post-Monsoon, Winter and Pre-Monsoon, respectively. Corresponding RMSE values were 0.0267, 0.0107, 0.0050 and 0.0073 m³/m³, while MAE values were 0.0158, 0.0049, 0.0027 and 0.0044 m³/m³. Rainfall was the dominant predictor during Monsoon, Post-Monsoon and Pre-Monsoon, whereas LST was most important during Winter. The annual RF model achieved an R² of 0.9272, compared with 0.6028 for the Multiple Linear Regression baseline. The results demonstrate that soil-moisture predictability and the relative importance of environmental controls vary markedly across seasons. Season-specific machine-learning modelling therefore provides a useful framework for characterising regional soil-moisture variability across Bihar using multi-source environmental observations.