Furness (/ˈfɜːrnɪs, fɜːrˈnɛs/ FUR-niss, fur-NESS) is a peninsula and region of Cumbria, England. Together with the Cartmel Peninsula it forms North Lonsdale, historically an exclave of Lancashire. On 1 April 2023 it became part of the new unitary authority of Westmorland and Furness.

The Furness Peninsula, also known as Low Furness, is an area of villages, agricultural land and low-lying moorland, with the industrial town of Barrow-in-Furness at its head. The peninsula is bordered by the estuaries of the River Duddon to the west and the River Leven in Morecambe Bay to the east. The wider region of Furness consists of the peninsula and the area known as High Furness, which is a relatively mountainous and sparsely populated part of England, extending inland into the Lake District and containing the Furness Fells. The inland boundary of the region is formed by the rivers Leven, Brathay and Duddon, and the lake of Windermere. Off the southern tip of Furness is Walney Island, 11 miles (18 kilometres) long, as well as several smaller islands.

Barrow-in-Furness, which developed when the Furness iron industry flourished in the 19th century, is the region's largest settlement, with a population of over 91,000. The remainder of Furness is predominantly rural, with Ulverston the only other settlement with more than 10,000 people. Much of High Furness consists of moorland, mountain or woodland environments.

The name, which is first recorded in 1150 as Fuththernessa, is interpreted as "headland by the rump-shaped island," from Old Norse futh (genitive futhar), meaning rump, and nes, meaning headland. The island in question may be Piel Island, with the name originally referring to the headland immediately opposite (where Rampside is), before being extended to the entire region. Alternatively it could be Walney Island: though it little resembles a rump today, erosion could have altered its shape over time.

Furness's border follows the River Duddon up to Wrynose Pass, and then the Brathay until it flows into Windermere. The mere forms most of the eastern boundary, with the rest being made up of the Leven, from its source at Windermere's southern tip to its mouth at Morecambe Bay. In total Furness has an area of about 250 square miles (650 square kilometres).

The Furness Fells are formed of Ordovician volcanic rocks, and Silurian shales and slates to the south. They are cut through by Windermere, Coniston Water, and numerous valleys which drain into the Esk, the Duddon, and Morecambe Bay. The higher ground is rocky heathland, with frequent tarns, while the lower ground supports pasture and woodland. In the east there are two main chains of hills: one overlooking Windermere, with Latterbarrow (245 m) as its highest point, and the other, which reaches 300 m, overlooking Coniston Water. Between them is flat country and Esthwaite Water. West of Coniston Water is the highest range, the Coniston Fells, with the Coniston Old Man (803 m) as its highest point (and historically the highest point in Lancashire). A lesser range extends from Torver to just north of Dalton, but south of that the landscape is flat; this area is also called Plain Furness. The low rolling hills of Low Furness are formed of glacial deposits, mainly boulder clay, above Triassic sandstone and Carboniferous limestone. There are large deposits of iron ore here, of very pure quality.

The human geography of the area is dominated by the town of Barrow-in-Furness. Barrow is located on the tip of the peninsula and Walney Island, and the Borough of Barrow-in-Furness, which includes the small town of Dalton-in-Furness has a population of 69,100. Barrow, which grew from a small village to a large town during the 19th century, is characterized by a grid pattern of streets of terraced houses, surrounded by more contemporary suburbs. Though the Royal Port of Barrow still contains much industry, there has also been significant redevelopment of former dock areas into retail parks, office blocks and spaces of light industry. The other major town in the area is Ulverston, at the border of Low and High Furness, and the population of Ulverston and its surrounding villages is 17,307. The corridor along the main A590 road between Ulverston and Barrow is relatively densely populated and urban. Despite decline, industry remains a bigger employer in this part of Furness than most of the UK, with BAE Systems, Kimberly-Clark (both Barrow) and GlaxoSmithKline (Ulverston) the largest employers. In Barrow and up the west coast of Furness, the Sellafield Nuclear site is also a significant employer. The rest of Furness is rural, with the mountainous, wooded and lacustrine environments. To the west, the main settlements of Kirkby-in-Furness and Broughton-in-Furness are dominated by farming and commuting. The Lake District National Park covers most of High Furness, with Coniston and Hawkshead the main centres for tourism, while major tourist attractions include Grizedale Forest, the Aquarium of the Lakes, South Lakes Safari Zoo and the Lakeside and Haverthwaite Railway. The northern and eastern communities of Furness share more in common with the Lake District towns of Ambleside and Bowness, outside of the region, than with the more urban areas of Low Furness. Mining was once a major part of the local economy, but today only the Burlington Slate Quarries at Kirkby remain of note.

The A590 is the main road into the region from the M6 Motorway. The A595 is the main route up the western coast of Furness, and the A593/A5084 run north-south from Low Furness to High Furness. The Furness Line provides railway connections to the West Coast Mainline through the south of Furness, and the Cumbria Coast Line is a slow rural branch line with services running north to Copeland and Carlisle. The Windermere Ferry in the east of Furness is England's only vehicular lake ferry.