Port Phillip, also referred to more academically as Port Phillip Bay, is a horsehead-shaped enclosed bay on the central coast of southern Victoria, Australia. The bay opens into the Bass Strait via a short, narrow channel known as The Rip, and is completely surrounded by localities of Victoria's two largest cities — metropolitan Greater Melbourne in the bay's main eastern portion north of the Mornington Peninsula, and the city of Greater Geelong in the much smaller western portion north of the Bellarine Peninsula. Geographically, the bay covers 1,930 square kilometres and the shore stretches roughly 264 km, with the volume of water around 25 cubic kilometres. Most of the bay is navigable, although it is extremely shallow for its size — the deepest portion is only 24 metres and half the bay is shallower than 8 m.
Before European settlement, the area around Port Phillip was divided between the territories of the Wathaurong, Wurundjeri and Boonwurrung Nations. Its waters and coast are home to seals, whales, dolphins, corals and many kinds of seabirds and migratory waders.