Altrincham (/ˈɒltrɪŋəm/ ^ⓘ OL-tring-əm, locally /ˈɒltrɪŋɡəm/) is a market town in Trafford, Greater Manchester, England, south of the River Mersey. It is 8 miles (13 km) southwest of Manchester, 3 miles (5 km) southwest of Sale and 10 miles (16 km) east of Warrington. At the 2021 census, the built up area had a population of 49,680.

It lies within the historic county boundaries of Cheshire, and became part of Greater Manchester in 1974. Altrincham developed as a market town following the right to hold a market being granted in 1290; the market continues today. Further socioeconomic development came with the extension of the Bridgewater Canal to Altrincham in 1765 and the arrival of the railway in 1849, stimulating industrial activity in the town. Outlying villages were absorbed by Altrincham's subsequent growth, along with the grounds of Dunham Massey Hall, formerly the home of the Earl of Stamford, and now a tourist attraction with three Grade I Listed Buildings and a deer park.

Altrincham has good transport links to Manchester, Sale, Stretford and Stockport among other destinations. The town has a strong middle-class presence: there has been a steady increase in Altrincham's middle classes since the 19th century. It is also home to Altrincham F.C. and three ice hockey clubs: Manchester Storm, Altrincham Aces and Trafford Tornados.

Local evidence of prehistoric human activity exists in the form of two Neolithic arrowheads found in Altrincham, and, further afield, a concentration of artefacts around Dunham. The remains of a Roman road, part of one of the major Roman roads in North West England connecting the legionary fortresses of Chester (Deva Victrix) and York (Eboracum), run through the Broadheath area. As it shows signs of having been repaired, the road was in use for a considerable period of time. The name Altrincham first appears as "Aldringeham", probably meaning "homestead of Aldhere's people". As recently as the 19th century it was spelt both Altrincham and Altringham.

Until the Normans invaded England, the manors surrounding Altrincham were owned by the Saxon thegn Alweard; after the invasion they became the property of Hamon de Massey, though Altrincham is not mentioned in the Domesday Book of 1086. The earliest documented reference to the town is from 1290, when it was granted its charter as a 'free borough' by Baron Hamon de Massey V. The charter, which still exists and is held by Trafford Council, allowed a weekly market to be held, and it is possible that de Massey established the town to generate income through taxes on trade and tolls. Altrincham was probably chosen as the site of the planned town rather than Dunham – which would have been protected by Dunham Castle – because its good access to roads was important for trade.

Altrincham Fair became St James's Fair or Samjam in 1319 and continued until 1895. Fair days had their own court of Pye Powder (a corruption of the French for "dusty feet"), presided over by the mayor and held to settle disputes arising from the day's dealings. By 1348 the town had 120 burgage plots – ownership of land used as a measure of status and importance in an area – putting it on a par with the Cheshire town of Macclesfield and above Stockport and Knutsford. The earliest known residence in Altrincham was "the Knoll", on Stamford Street near the centre of the medieval town. A 1983 excavation on the demolished building, made by South Trafford Archaeological Group, discovered evidence that the house dated from the 13th or 14th century, and that it may have contained a drying kiln or malting floor. During the English Civil War, men from Altrincham fought for the Parliamentarian Sir George Booth. During the war, armies camped on nearby Bowdon Downs on several occasions.

In 1754, a stretch of road south of Altrincham, along the Manchester to Chester route, was turnpiked. Turnpikes were toll roads which taxed passengers for the maintenance of the road. Further sections were turnpiked in 1765 from Timperley to Sale, and 1821 from Altrincham to Stockport. The maintenance of roads passed to local authorities in 1888, although by then most turnpike trusts had already declined. The connection of the Bridgewater Canal to Altrincham in 1765 stimulated the development of market gardening, and for many years Altrincham was noted for its vegetables. By 1767, warehouses had been built alongside the canal at Broadheath, the first step in the development of Broadheath as an industrial area and the beginning of Altrincham's industrialisation. The canal was connected in 1776 to the River Mersey, providing the town not only with a water route to Manchester, but also to the Irish Sea.

Moves to connect the town to the UK's railway network gained pace in 1845, when the Act of Parliament for the construction of the Manchester South Junction and Altrincham Railway (MSJAR) was passed. The first train left Altrincham early on 20 July 1849, carrying 65 passengers. The MSJAR had two stations in the town: Altrincham, on Stockport Road, and Bowdon – though not actually in Bowdon – on Lloyd Street/Railway Street. Both were replaced in 1881 by Altrincham & Bowdon railway station on Stamford New Road. The London and North Western Railway's station at Broadheath, on the town's northern edge, was opened in 1854, while a further connection was created on 12 May 1862 by the Cheshire Midland Railway (later the Cheshire Lines Committee), who opened their line from Altrincham to Knutsford.