St George's Channel (Welsh: Sianel San Siôr, Irish Muir Bhreatan) is a channel connecting the Irish Sea to the north and the Atlantic Ocean to the southwest. Historically, the name encompassed all the waters between Ireland in the west, and Wales and the West Country in the east; thus the Bristol Channel opened into Saint George's Channel. The name is now usually applied only to the waters near the narrowest part of the channel, between Carnsore Point in Wexford and Saint David's Head in Pembrokeshire. The sea to the north is now simply considered the South Irish Sea; the sea to the south is considered part of the Celtic Sea. However, it remains common in Ireland to talk about a cross-channel trip, cross-channel soccer, etc., where "cross-channel" means "to/from Great Britain".

Cape Cornwall at the south-western tip of Cornwall is where the St George's Channel meets the English Channel (a cape is where two channels of water meet). The St George's Channel coastline is the coastline of North Cornwall and North Devon in the West Country Pembrokeshire and Cardigan Bay in Wales and counties Wexford, Waterford and Cork in southern Ireland.

Regular visitors to the St George's Channel coast include basking sharks, dolphins, porpoises, Atlantic grey seals, leather-back turtles, lobsters and other impressive marine life. They attracted by the clean, clear waters and good food supplies of the coastlines of the West Country, Wales and South-West Ireland. North Cornwall, around Porthcurno and St Ives is famous for its regular spottings of basking sharks, while Pembrokeshire and Cardigan Bay is internationally recognised as a haven for bottle nosed dolphins.

The name is said to derive from a 14th-century legend that Saint George had voyaged to Roman Britain from the Byzantine Empire. The legend said he approached Britain via the channel that bears his name.