Precipitation isotopes integrate the cumulative effects of moisture transport, rainout, and land–atmosphere interactions, making them valuable tracers of monsoon hydrological processes. However, the dominant controls on isotope variability differ across the Indian Summer Monsoon region. Motivated by the contrasting long-term evolution of vertically integrated moisture transport over the Arabian Sea and the Bay of Bengal (BOB), we compare decade-long precipitation isotope records from the Monsoon Core Zone (MCZ) of India and the BOB to investigate their responses to contrasting moisture transport regimes. The BOB record exhibits a strong relationship between precipitation δ18O and large-scale moisture transport and convergence, consistent with progressive isotopic depletion during moisture advection and rainout. In contrast, isotope variability over the MCZ is controlled primarily by recycled rainfall rather than total precipitation, highlighting the dominant influence of land-atmosphere moisture recycling. Kernel density analyses reveal distinct regional distributions of δ18O, whereas d-excess distributions largely overlap, indicating similar large-scale moisture-source conditions despite contrasting moisture-processing pathways. Together, these findings identify a transition from a moisture-convergence-dominated oceanic isotope regime to a recycling-dominated continental regime. Because precipitation δ18O over the MCZ responds more strongly to recycled rainfall than to primary rainfall, our results suggest that speleothem δ18O records from central India may reflect changes in terrestrial moisture recycling in addition to monsoon rainfall intensity, with implications for how their paleoclimatic interpretation is developed.