Being an integral effect of sub-seasonal rain spells over the season, the seasonal mean south Asian monsoon (SAM) rainfall could be affected by change in the length of the rainy season (LRS). An objective definition of the duration of the SAM season has, however, been lacking. Here we show that the meridional gradient of tropospheric temperature (ΔTT) over the SAM region controls the LRS and defines the SAM season. It is further shown that ENSO induces decreased SAM rainfall by regulating the LRS. The atmospheric response to tropical sea surface temperature (SST) over the tropical Pacific during an evolving El Niño reduces ΔTT over the SAM region and shortens LRS by delaying the onset and advancing the withdrawal. The strong negative correlation between LRS and ENSO related SST has remained steady and provides basis for improved prediction of seasonal mean SAM rainfall variability.