We review the skill of thirty coupled climate models participating in the Coupled Model Intercomparison Project 5 (CMIP5) in terms of reproducing properties of the seasonal cycle of precipitation over the major river basins of South and Southeast Asia (Indus, Ganges, Brahmaputra and Mekong) for the historical period . We also present projected changes by these models by the end of century (2061-2100) under the extreme scenario RCP8.5. First, we assess their ability to reproduce observed timings of the monsoon onset and the rate of rapid fractional accumulation (RFA slope) -a measure of seasonality within the active monsoon period. Secondly, we apply a threshold-independent seasonality index (SI) -a multiplicative measure of precipitation (P) and extent of its concentration relative to the uniform distribution (relative entropy -RE). We apply SI distinctly for the whole monsoonal precipitation regime (MPR), westerly precipitation regime (WPR) and annual precipitation regime. For the present climate, neither any single model nor the multi-model mean performs best in all chosen metrics. Models show overall a modest skill in suggesting right timings of the monsoon onset while the RFA slope is generally underestimated. One third of the models fail to capture the monsoon signal over the Indus basin. Mostly, SI estimates for WPR are simulated higher than observed for all basins, while for MPR, it is simulated higher (lower) for the Ganges and Brahmaputra (Indus and Mekong) basins, following the pattern of overestimation (underestimation) of precipitation. However, models are biased positive (negative) for RE estimates over the Indus and Ganges (Brahmaputra and Mekong) basins, implying the extent of precipitation concentration for MPR and number of dry days within WPR higher (lower) than observed for these basins. Under the RCP8.5 scenario, most of the models project a slightly delayed monsoon onset, and a general increase in the RFA slope, precipitation and extent of its concentration (RE), all suggesting a higher seasonality of the future MPR for all basins. Similarly, a modest inter-model agreement suggests a less intermittent WPR associated with a general decrease in number of wet days and a decrease (increase) in precipitation over the Indus and Ganges (Brahmaputra and Mekong) basins. Based on SI, multi-model mean suggests an extension of the monsoonal domain westward over northwest India and Pakistan and northward over China. These findings have serious implications for the food and water security of the region in the future.