The Bristol Channel (Welsh: Môr Hafren, lit. 'Severn Sea') is a major inlet in the island of Great Britain, separating South Wales (from Pembrokeshire to the Vale of Glamorgan) and South West England (from Devon to North Somerset). It extends from the smaller Severn Estuary of the River Severn (Welsh: Afon Hafren) to the North Atlantic Ocean. It takes its name from the English city and port of Bristol.

Long stretches of both sides of the coastline are designated as Heritage Coast. These include Exmoor, Bideford Bay, the Hartland Point peninsula, Lundy Island, Glamorgan, Gower Peninsula, Carmarthenshire, South Pembrokeshire and Caldey Island.

Until Tudor times the Bristol Channel was known as the Severn Sea, and it is still known as this in both Welsh: Môr Hafren and Cornish: Mor Havren.

The International Hydrographic Organization (IHO) defines the offshore western limit of the Bristol Channel as "a line joining Hartland Point in Devon (51°01′N 4°32′W / 51.017°N 4.533°W / 51.017; -4.533) to St. Govan's Head in Pembrokeshire (51°36′N 4°55′W / 51.600°N 4.917°W / 51.600; -4.917)". Western and northern Pembrokeshire, north Cornwall and that part of Devon west of Hartland Point are outside the IHO defined limits of the Bristol Channel, and are considered part of the seaboard of the Atlantic Ocean, specifically the Celtic Sea. Lundy lies on the boundary between the Bristol Channel and the Atlantic. In common usage, the distinction between the Bristol Channel and Celtic Sea is less clearly defined, with the name Bristol Channel sometimes applied to areas of the Celtic Sea.

Upstream, the distinction between the Bristol Channel and Severn Estuary is more ambiguous, with no official boundary between them. Some sources consider the Bristol Channel to encompass the estuary, or draw no distinction between them. The Severn Estuary Partnership encompasses an area upstream of Hurlstone Point and Nash Point, while the Living Levels Partnership draw the line between Lavernock Point and Sand Point, marked by Steep Holm and Flat Holm islands. Lavernock Point is also used as the boundary between Marine Character Areas 28 and 29, named Bristol Channel and Severn Estuary. In common usage, the term Bristol Channel is often applied to areas upstream of these lines.

The Bristol Channel–Severn Estuary system extends eastward inland to the limit of tidal influence, at Gloucester Docks. The channel shoreline alternates between resistant and erosional cliff features, interspersed with depositional beaches backed by coastal sand dunes; in the Severn Estuary, a low-lying shoreline is fronted by extensive intertidal mudflats. The Severn Estuary and most of the embayments around the channel are less than 30 ft (9 m) deep. Within the channel, however, there is an east–west valley 65–100 ft (20–30 m) deep, which is thought to have been formed by fluvial run-off during Pleistocene phases of lower sea level. Along the margins of the Bristol Channel are extensive linear tidal sandbanks, which are dredged for aggregates. In the outer Bristol Channel, off the Welsh coast, are the OBel Sands, an extensive area of sand waves up to 62 ft (19 m) high, covering more than 400 sq mi (1,036 km²)).

The Bristol Channel is an important area for wildlife, in particular waders, and has protected areas, including national nature reserves such as Bridgwater Bay at the mouth of the River Parrett. At low tide large parts of the channel become mud flats due to the tidal range of 43 ft (13 m), frequently stated to be second only to the Bay of Fundy in Eastern Canada but smaller than those at Ungava Bay, also in Canada. Development schemes have been proposed along the channel, including an airport and a tidal barrier for electricity generation, but conservation issues have so far managed to block such schemes.

The largest islands in the Bristol Channel are Lundy, Steep Holm and Flat Holm, which are mostly uninhabited and protected as nature reserves, and are home to some unique wild flower species. In 1971 a proposal was made by the Lundy Field Society to establish a marine reserve. Provision for the establishment of statutory Marine Nature Reserves was included in the Wildlife and Countryside Act 1981, and on 21 November 1986 the Secretary of State for the Environment announced the designation of a statutory reserve at Lundy. There is an outstanding variety of marine habitats and wildlife, and a large number of rare and unusual species in the waters around Lundy, including some species of seaweed, branching sponges, sea fans and cup corals.