The El Niño–Southern Oscillation (ENSO) and Indian monsoon have undergone significant interdecadal changes in variance and coherency over the last 125 years. Wavelet analysis of Niño3 SST, Southern Oscillation Index (SOI), and all-India rainfall shows intervals of high ENSO–monsoon variance (1875–1920 and 1960–90) and low variance (1920–60). The ENSO–monsoon variance also contains a 12–20-yr modulation of amplitudes. Annual-cycle variance is negatively correlated with interannual ENSO signal, with higher annual-cycle variance during 1935–60. Wavelet coherency shows high coherence between Niño3 SST and Indian rainfall, especially during high variance intervals. The two are approximately 180° out of phase, with a gradual increase in phase difference over time. Results are robust across datasets and methods. Interdecadal changes in ENSO–monsoon variance and coherency are independent of datasets and analysis methods. The results suggest three possible explanations for these changes: internal variability of the tropical climate system, external changes in background climate, or a combination of internal and external factors. The findings highlight the importance of understanding ENSO–monsoon variability for improving forecasts. The study underscores the need for further research into the causes of interdecadal changes and their implications for climate predictability.The El Niño–Southern Oscillation (ENSO) and Indian monsoon have undergone significant interdecadal changes in variance and coherency over the last 125 years. Wavelet analysis of Niño3 SST, Southern Oscillation Index (SOI), and all-India rainfall shows intervals of high ENSO–monsoon variance (1875–1920 and 1960–90) and low variance (1920–60). The ENSO–monsoon variance also contains a 12–20-yr modulation of amplitudes. Annual-cycle variance is negatively correlated with interannual ENSO signal, with higher annual-cycle variance during 1935–60. Wavelet coherency shows high coherence between Niño3 SST and Indian rainfall, especially during high variance intervals. The two are approximately 180° out of phase, with a gradual increase in phase difference over time. Results are robust across datasets and methods. Interdecadal changes in ENSO–monsoon variance and coherency are independent of datasets and analysis methods. The results suggest three possible explanations for these changes: internal variability of the tropical climate system, external changes in background climate, or a combination of internal and external factors. The findings highlight the importance of understanding ENSO–monsoon variability for improving forecasts. The study underscores the need for further research into the causes of interdecadal changes and their implications for climate predictability.