High Quality Content by WIKIPEDIA articles! The South Bay is a subregion of the San Francisco Bay Area, California, United States and is roughly synonymous with Silicon Valley, the Santa Clara Valley, and Santa Clara County (in general). It is the most populous and most urbanized part of the Bay Area. The South Bay is one of several Subregions in the Bay Area including: San Francisco, the North Bay including parts of the Wine Country, the Peninsula, and the East Bay, among others.San Jose is the largest city in the South Bay and the Bay Area.the largest city in the South Bay and the Bay Area.