A new objective approach is proposed to identify the seasonal cycle of rainfall for major monsoon regions of the globe. Mean sea level pressure and low-level zonal wind are combined to establish a standardized monsoon index. The index is able to determine the timing of the summer monsoon rainy season across all major monsoon regions in Asia-Australia, Africa, and the Americas. A detailed application of the index in detecting the monsoon onset-withdrawal phases is demonstrated for the Vietnam-South China Sea region. A significant advantage of the index is that it can be used both for studies of monsoon rainfall variability and for monsoon predictability at seasonal time scales.