The River Mersey (/ˈmɜːrzi/) is a major river in North West England. Its name derives from Old English and means "boundary river", possibly referring to its having been a border between the ancient kingdoms of Mercia and Northumbria. For centuries it has formed part of the boundary between the historic counties of Lancashire and Cheshire.

The Mersey is currently regarded as starting at the confluence of the River Tame and River Goyt in the centre of Stockport. However, older definitions, and many older maps, place its start at the confluence of the Etherow and Goyt, near Marple, about 4 miles (6.4 km) further upstream Even as late as its 1911 edition, the Encyclopædia Britannica stated that the River Mersey "is formed by the junction of the Goyt and the Etherow a short distance below Marple in Cheshire on the first-named stream."

It flows westwards through south Manchester, then into the Manchester Ship Canal near Irlam Locks, becoming a part of the canal and maintaining its water levels. After 4 miles (6.4 km) it exits the canal, flowing towards Warrington where it widens. It then narrows as it passes between Runcorn and Widnes. The river widens into a large estuary, which is 3 miles (4.8 km) across at its widest point near Ellesmere Port. The course of the river then turns northwards as the estuary narrows between Liverpool and Birkenhead on the Wirral Peninsula to the west, and empties into Liverpool Bay. In total the river flows 69 miles (111 km).

Part of the Mersey Railway, a rail tunnel between Birkenhead and Liverpool opened in 1886. Two road tunnels pass under the estuary from Liverpool: the Queensway Tunnel opened in 1934 connecting the city to Birkenhead, and the Kingsway Tunnel, opened in 1971, to Wallasey. A road bridge, completed in 1961 and later named the Silver Jubilee Bridge, crosses between Runcorn and Widnes, adjacent to the Runcorn Railway Bridge which opened in 1868. A second road bridge, the Mersey Gateway, opened in October 2017, carrying a six-lane road connecting Runcorn's Central Expressway with Speke Road and Queensway in Widnes. The Mersey Ferry operates between the Pier Head in Liverpool and Woodside in Birkenhead and Seacombe, and has become a tourist attraction offering cruises that provide an overview of the river and surrounding areas.

Water quality in the Mersey was severely affected by industrialisation, and in 1985, the Mersey Basin Campaign was established to improve water quality and encourage waterside regeneration. In 2009 it was announced that the river is "cleaner than at any time since the industrial revolution" and is "now considered one of the cleanest in the UK". The Mersey Valley Countryside Warden Service manages local nature reserves such as Chorlton Ees and Sale Water Park.

The river gave its name to Merseybeat, developed by bands from Liverpool, notably the Beatles. In 1965 it was the subject of the top-ten hit single "Ferry Cross the Mersey" by Gerry and the Pacemakers.

The name "Mersey" is derived from the Anglo-Saxon mǣres, 'of a boundary' and ēa, 'a river'. The Mersey was possibly the 'border river' between Mercia and Northumbria. Its Welsh name is Afon Merswy. Prior to Anglo-Saxon conquest of the region, the river was almost certainly known by a Celtic name - recorded by Ptolemy as "Seteia" (as the Southern border of the Brittonic "Setantii" tribe). "Seteia" and "Setantii" both lack clear Celtic roots, leading some to suggest that this was likely a corruption of the actual name. Some, including David Parsons, have suggested the original forms "Segeia" and "Segantii" - based on the root "sego" meaning "strong". Andrew Breeze instead suggests the original forms "Meteia" and "Metantii" - from "met" meaning "cut" or "harvest" - with the likely meaning being that the Metantii were "reapers (of men), cutters-down (of enemies)", with the river Meteia meaning "reaping one" or "she that cuts down" (Breeze notes the possibility of a local deity associated with the river, but highlights that this is only a supposition based on known deities like Sulis and Aerfen).

The Mersey is formed from three tributaries: the River Etherow, the River Goyt and the River Tame. The modern accepted start of the Mersey is at the confluence of the Tame and Goyt, in central Stockport, Greater Manchester. However, older definitions, and many older maps, place its start a few miles up the Goyt at Compstall; for example the 1911 Encyclopædia Britannica states "It is formed by the junction of the Goyt and the Etherow a short distance below Marple in Cheshire on the first-named stream." The 1784 John Stockdale map shows the River Mersey extending to Mottram, and forming the boundary between Cheshire and Derbyshire. In the west of Stockport it flows at the base of a cliff below the road called Brinksway before reaching flat country. An early Ordnance Survey map indicates the spring at Red Hole on the border of Cheshire and Yorkshire at the head of one of the Etherow's longest headwaters, as being the Source of the River Mersey.