Monitoring agricultural fallow lands is crucial for regional agro-economic growth, as it enables potential crop intensification by utilizing these lands in the seasons apart from the primary growing season. This study attempts to map the fallow lands in the Rabi season in Jharkhand during 2015–2024 through a spectral index threshold-based approach. Each year’s seasonal maximum Normalized Difference Vegetation Index (NDVI) was derived from the Landsat 8 time-series datasets. Sample points collected over the fallow lands in 2015 were used to estimate the NDVI threshold based on the statistical values of the points. The lower and upper thresholds to map fallow lands were derived at 0.214 and 0.312, respectively. The results reveal that the maximum lands were left uncultivated in 2015 (30.86% of the available agricultural area) and 2018 (25.61% of the agricultural area) due to the scanty amount of monsoon and post-monsoon rainfall in the previous growing seasons. The fallow land map of 2015 achieved an 88% accuracy when validated with the reference land use land cover data. Lohardaga and Giridih districts were identified as having a maximum extent of fallow lands, with more than 50% of the agricultural area left uncultivated in these two years. The outcomes of this study would help in understanding the cropping system dynamics of the region and identify the potential lands to grow crops in secondary growing seasons like Rabi, further improving crop productivity and ensuring food security.