St Austell Bay (Cornish: Baya Ti war Dreth)^[1] is a bay on Cornwall's south coast which is bounded to the east by Gribbin Head and to the west by Black Head.^[2]

Since 1 April 2009, it has also been the name of a civil parish, one of four new parishes created on for the St Austell area. It lies southeast of the town of St Austell^[2] and stretches along the coast from Charlestown in the north to Black Head in the south. It includes the communities of Charlestown, Duporth, Porthpean (Higher and Lower Porthpean) and Trenarren and is represented by seven councillors.^[3]

In 2023, a 887-acre (359 ha) seagrass meadow, one of the largest in the UK, was discovered off the coast of St Austell Bay. Divers which were sent in to examine the site recorded 56 species living there, which included short-snouted seahorses and broadnosed pipefish. The divers also examined beds of maerl a little further out from the coast, where they found an additional 66 species, including the curled octopus and streaked gurnard.^[4]