Abstract A phenomenon referred to here as Java–Sumatra Niño/Niña (JSN or JS Niño/Niña) is characterized by the appearance of warm/cold sea surface temperature anomalies (SSTAs) in the coastal upwelling region off Java–Sumatra in the southeastern equatorial Indian Ocean. JSN develops in July–September and sometimes as a precursor to the Indian Ocean dipole, but often without corresponding SSTAs in the western equatorial Indian Ocean. Although its spatiotemporal evolution varies considerably between individual events, JSN is essentially an intrinsic mode of variability driven by local atmosphere–ocean positive feedback, and thus does not rely on remote forcing from the Pacific for its emergence. JSN is an important driver of climate variability over the tropical Indian Ocean and the surrounding continents. Notably, JS Niña events developing in July–September project onto the South and Southeast Asian summer monsoons, increasing the probability of heavy rainfall and flooding across the most heavily populated regions of the world.