False Bay (Afrikaans: Valsbaai) is a body of water in the Atlantic Ocean between the mountainous Cape Peninsula and the Hottentots Holland Mountains in the extreme south-west of South Africa. The mouth of the bay faces south and is demarcated by Cape Point to the west and Cape Hangklip to the east. The north side of the bay is the low-lying Cape Flats, and the east side is the foot of the Hottentots Holland Mountains to Cape Hangklip, which is at nearly the same latitude as Cape Point. In plan, the bay is approximately square, with roughly the same extent from north to south as from east to west, and the southern side open to the ocean. The seabed generally slopes gradually down from north to south, and is mostly fairly flat unconsolidated sediments. Much of the bay is off the coast of the City of Cape Town, and it includes part of the Table Mountain National Park Marine Protected Area and the whole of the Helderberg Marine Protected Area. The name "False Bay" was applied at least 300 years ago by sailors returning from the east, who confused Cape Point and Cape Hangklip, which are somewhat similar in profile when approached from the southeast.

False Bay is at the extreme western end of the inshore Agulhas marine ecoregion which extends from Cape Point to the Mbashe river over the continental shelf, in the overlap zone between Cape Agulhas and Cape Point where the warm Agulhas Current and the cooler South Atlantic waters mix. The continental shelf is at its widest in this ecoregion, extending up to 240 km (150 mi) offshore on the Agulhas Bank, but is considerably narrower off False Bay. This ecoregion has the highest number of South African marine endemics and is a breeding area for many species. The transition between the Agulhas ecoregion and the cooler Benguela ecoregion is at Cape Point, on the western boundary of False Bay.

False Bay also contains South Africa's largest naval base at Simon's Town (historically a base for the Royal Navy), and small fishing harbours at Kalk Bay and Gordon's Bay.

The western side is bordered by the Cape Peninsula, and this stretch of coastline includes the smaller Buffels Bay, Smitswinkel Bay, Simon's Bay, and Fish Hoek Bay. At Muizenberg, the coastline becomes relatively low and sandy and curves east across the southern boundary of the Cape Flats to Gordon's Bay to form the northern boundary of False Bay. From Gordon's Bay, the coastline swings roughly south and zigzags along the foot of the Hottentots Holland Mountains to Cape Hangklip, which is at nearly the same latitude as Cape Point. The highest peak on this side is Kogelberg at 1,269 m (4,163 ft).

In plan, the bay is approximately square, with rather wobbly edges, roughly the same extent from north to south as from east to west (30 km), with the entire southern side open to the ocean. The area of False Bay has been measured at about 1,090 km² (420 sq mi), and the volume is approximately 45 km³ (11 cu mi) (average depth about 40 m). The land perimeter has been measured from a 1:50,000-scale map at 116 km.

The eastern and western shores of the bay are very rocky and even mountainous; in places, large cliffs plunge into the water. Notable peaks associated with the bay include Koeëlberg (1,289 m (4,229 ft)), which rises from the water itself, forming the highest point of the Kogelberg, as well as Somerset Sneeukop (1590m / 5217 feet) and Wemmershoek Peak (1,788 m (5,866 ft)), which are clearly visible across the bay. Some of the highest peaks visible across False Bay include Du Toits Peak near Paarl (1,995 m (6,545 ft)), Klein Winterhoek Peak near Tulbagh (1,995 m (6,545 ft)), Mostertshoek Peak at the Western extreme of the Michell's Pass (2,008 m (6,588 ft)) and Groot Winterhoek Peak North of Tulbagh (2,077 m (6,814 ft)). The northern shore is defined by a very long, curving, sandy beach. This sandy northern perimeter of the bay marks the southern edge of the Cape Flats. The bay is 30 km wide at its widest point.

Suburbs of Cape Town stretch right across the Cape Flats from Simon's Town halfway down the Cape Peninsula to the north-eastern corner at Gordon's Bay. There are also two small towns of the Overberg region on the east coast of the bay, Rooiels and Pringle Bay.

Coastal landmarks visible from offshore in False Bay, listed clockwise from Cape Point to Cape Hangklip: