Rainfall variability in the Asia-Pacific region is largely influenced by the El Niño Southern Oscillation (ENSO), the Interdecadal Pacific Oscillation (IPO), the Intertropical Convergence Zone (ITCZ), and the Indian Ocean Dipole (IOD). Of particular interest is the ENSO phenomenon, which provides the basis for seasonal prediction of rainfall using statistical methods employing atmospheric (Southern Oscillation Index or SOI) and sea surface temperature anomaly (SSTa) data as proxies for ENSO. In this paper, we used several derivatives of ENSO-based predictors to assess rainfall forecast skill across the study region and the impact of ENSO on the onset and duration of the monsoon. The results show that ENSO has a significant skill in predicting seasonal rainfall and monsoon onset (and duration) in the Philippines and Indonesia. The strength of relationship in Bangladesh was weak with little potential at present for developing an operational seasonal climate prediction model based on ENSO.