Mosman Bay is a bay of Sydney Harbour adjacent to the suburb of Mosman, 4 km north-east of the Sydney CBD in New South Wales, Australia. Two of Mosman's three ferry wharves, Mosman Bay and South Mosman are within the bay, both being served by the Mosman loop ferry service.
In 1789 HMS Sirius entered what is now known as Mosman Bay. Originally known as Great Sirius Cove, this name lives on in the next bay to the east, Sirius Cove (originally Little Sirius Cove). In 1831, the bay's current namesake, Archibald Mosman, obtained a land grant for the area surrounding the bay. Together with his twin brother George, Mosman founded a whaling station within Mosman Bay.