The Indian Summer Monsoon (ISM) underpins water security, agriculture, and economic stability across the subcontinent. This study reassesses its 21st-century behavior using authenticated secondary data: the IITM All-India Summer Monsoon Rainfall (AISMR) percent departures (1871–2019) and the NOAA/CPC ERSSTv5 Niño-3.4 monthly sea-surface temperature anomalies aggregated to JJAS (1950–2019). Methods include Mann–Kendall and Sen’s slope for monotonic trends, Pearson and Kendall correlation for ENSO teleconnections, phase-wise (El Niño/Neutral/La Niña) composites, and 21-year rolling correlations to probe non-stationarity. Results show no robust century-scale trend in national-mean JJAS rainfall; interannual variability dominates. AISMR is significantly and inversely related to JJAS Niño-3.4, though the strength of this relationship varies by epoch. Era-wise distributions indicate heavier tails as more frequent moderate deficits and surpluses despite a near-stationary mean. A literature-anchored synthesis (IMD daily grids, ERA5, and peer-reviewed studies) corroborates growing regional contrasts and an increase in heavy-rain events in parts of central/western India, even as some northeastern and northwestern regions show declines. Policy implications emphasize planning for volatility via impact-based forecasts, adaptive water operations, climate-smart agriculture, and updated urban drainage design. The analysis is transparent and reproducible, with figures and tables generated programmatically from the cited sources.