Santa Monica Bay is an arm of the Pacific Ocean in southern California, United States. Its boundaries are slightly ambiguous, but it is generally considered to be the part of the Pacific within an imaginary line drawn between Point Dume, in Malibu, and the Palos Verdes Peninsula. Its eastern shore forms the western boundary of the Los Angeles Westside and South Bay regions. Although it was fed by the Los Angeles River prior to the river's catastrophic change in course in 1825, the only stream of any size now flowing into it is Ballona Creek. Other waterways draining into the bay include Malibu Creek, and Topanga Creek.

In the 1930s, gambling ships anchored beyond the three mile limit, then measured from the beach. The ships were popular and a fleet of ever-larger ships and barges appeared until the State Attorney General got the limit recalculated to exclude the bay. The largest ship held off the state police for nine days with submachine guns in what the newspapers called The Battle of Santa Monica Bay.

Once a major commercial fishery, Santa Monica Bay's water quality declined drastically in the 20th century as development of Los Angeles County resulted in large amounts of sewage and trash-rich storm runoff being dumped into its waters. Through restoration projects mandated by the Clean Water Act and advocated by groups such as Heal the Bay, the bay's water quality has improved fairly dramatically from its early-1980s nadir. Hyperion sewage treatment plant's output is now far cleaner than it had been. However, during the region's rainy winters, it still suffers from algal bloom and other water pollution-related maladies, forcing the closure of most of the famous beaches along its shore.