Ralphs Bay is a body of water in south-east Tasmania, Australia. It is surrounded by the Tranmere and South Arm peninsulas. Sea access to the bay is from the River Derwent.
Ralphs Bay is a shallow, windy bay, situated on the south-east of the Derwent Estuary, about 12 kilometres from the south-east of Hobart in Tasmania, Australia. The bay is sheltered by the low, grassy hills of Droughty Point to the north-west and the spit of South Arm and Opossum Bay to the southwest.
The red-necked stint migrates from Siberia every year and most of the birds, which do that, find themselves in Ralphs Bay. It is also an important area for the spotted handfish. They are critically endangered and, because of the pollution in the River Derwent, live in majority at Ralphs Bay and its surrounding waters.
The bay is part of the South Arm Important Bird Area (IBA), identified as such by BirdLife International because of its importance for the conservation of pied oystercatchers and of the migratory waders, or shorebirds, of the East Asian – Australasian Flyway.
Since 2004 Ralphs Bay has been the focus of a local development debate. Walker Corporation had planned a major development in the bay near Lauderdale. The proposal included a canal development and a marina, to be known as "Lauderdale Quay". It met strong opposition from the local community which was concerned that the development would destroy native wildlife habitat in the area and reduce public access to the foreshore.