Chat Moss is a large area of peat bog that makes up part of the City of Salford, Metropolitan Borough of Wigan and Trafford in Greater Manchester, England. It also makes up part of Metropolitan Borough of St Helens in Merseyside and Warrington in Cheshire. North of the Manchester Ship Canal and River Mersey, 5 miles (8 km) to the west of Manchester, it occupies an area of about 10.6 square miles (27.5 km²).

As it might be recognised today, Chat Moss is thought to be about 7,000 years old, but peat development seems to have begun there with the ending of the last ice age, about 10,000 years ago. The depth of peat ranges from 24 to 30 feet (7 to 9 m). A great deal of reclamation work has been carried out, particularly during the 19th century, but a large-scale network of drainage channels is still required to keep the land from reverting to bog. In 1958 workers extracting peat discovered the severed head of what is believed to be a Romano-British Celt, possibly a sacrificial victim, in the eastern part of the bog near Worsley.

Much of Chat Moss is now prime agricultural land, although farming in the area is in decline. A 228-acre (92 ha) area of Chat Moss, notified as Astley and Bedford Mosses, was designated a Site of Special Scientific Interest in 1989. Along with nearby Risley Moss and Holcroft Moss, Astley and Bedford Mosses has also been designated as a European Union Special Area of Conservation, known as Manchester Mosses.

Chat Moss threatened the completion of the Liverpool and Manchester Railway, until George Stephenson, with advice from East Anglian marshland specialist Robert Stannard, succeeded in constructing a railway line through it in 1829; his solution was to "float" the line on a bed of bound heather and branches topped with tar and covered with rubble stone. The M62 motorway, completed in 1976, crosses the bog, to the north of Irlam. Also the A580 crosses the bog, forming Leigh, Lowton and Astley's (Wigan MBC)'s boundary with Warrington, Culcheth and Glazebury, Croft, and Kenyon.

Chat Moss may be named after St Chad, a 7th-century bishop of Mercia, but as it was once part of a great tree-edged lake, as evidenced by the numerous wood remains in the lower levels of the peat, it is perhaps more likely that the name stems from the Celtic word ced, meaning wood. Chat Moss could also derive from Ceatta, an Old English personal name and mos, a swamp or alternatively the first element could be the Old English ceat meaning a piece of wet ground. It was recorded as Catemosse in 1277 and Chatmos in 1322. Moss is the local name for a peat bog.

Daniel Defoe visited the area in 1724, on his way from Warrington to Manchester:

From hence (Warrington), on the road to Manchester, we pass'd the great bog or waste call'd Chatmos, the first of that kind that we see in England ... The surface, at a distance, looks black and dirty, and is indeed frightful to think of, for it will bear neither horse or man, unless in an exceeding dry season, and then not so as to be passable, or that any one should travel over them ... What nature meant by such a useless production, 'tis hard to imagine; but the land is entirely waste, excep ... for the poor cottagers fuel, and the quantity used for that is very small.

— Daniel Defoe, A tour thro' the whole island of Great Britain, divided into circuits or journies