<p>The Indian Summer Monsoon (ISM) is the primary contributor of rainfall in the Indo-Gangetic Plains and therefore is of great relevance to the rain-fed agriculture in the region. The hydroclimate dynamics across the Indo-Gangetic plains, despite being critical to the socio-economy of the region, are not well understood. Oxygen- and hydrogen-isotope analyses of precipitation have served as a powerful tool to understand hydroclimate processes. Existing studies have examined the isotopic composition of meteoric water across the Indian subcontinent, but systematic investigation of spatial patterns on the Indo-Gangetic Plains has not been reported. In this study, we present analysis of stable isotopes of rainwater collected weekly, simultaneously from different locations along an east-west transect across the eastern part of the Indo-Gangetic Plains for the 2023 summer monsoon season. Our research findings suggest significant variations in rainwater isotopic values during the summer monsoon season across the Indo-Gangetic Plains. A trend towards lighter oxygen and hydrogen isotope values in rainwater is observed from June to September across all zones, with slightly heavier isotope values during August. However, there is no strong correlation between rainfall amount and isotopic values across the zones. Instead, the isotopic variations show good correlation with the Outgoing Longwave Radiation (OLR) values on a monthly scale. This rainfall variability is associated with the dry and wet phase of the Madden-Julian Oscillation. Correlation analysis between rainwater isotopes and meteorological parameters in concert with back-trajectory simulations demonstrates spatial changes in moisture sources throughout the summer monsoon period over the Indo-Gangetic plains.</p>