Sleaford is a market town and civil parish in the North Kesteven district of Lincolnshire, England. On the edge of the Fenlands, it is 11 miles (18 kilometres) north-east of Grantham, 16 mi (26 km) west of Boston, and 17 mi (27 km) south of Lincoln. It is the largest settlement in North Kesteven with a population of 19,807 in 2021. Centred on the former parish of New Sleaford, the modern boundaries and urban area include Quarrington to the south-west, Holdingham to the north-west and Old Sleaford to the east. The town is bypassed by the A17 and the A15 roads. Sleaford railway station is on the Nottingham to Skegness (via Grantham) and Peterborough to Lincoln lines.

The first settlement formed in the Iron Age where a prehistoric track crossed the River Slea. It was likely home to a mint for the Corieltauvi in the 1st centuries BC and AD. Evidence of Roman and Anglo-Saxon settlement has been found. Medieval records differentiate between Old and New Sleaford, the latter emerging by the 12th century around the present-day market place and St Denys' Church; Sleaford Castle was also built at that time for the Bishops of Lincoln, who owned the manor. Granted the right to hold a market in the mid-12th century, New Sleaford developed into a market town and became locally important in the wool trade, while Old Sleaford (based near the site of the prehistoric settlement) declined.

From the 16th century, the landowning Carre family kept tight control over the town – it grew little in the early modern period. The manor passed by marriage to the Hervey family (Earls and later Marquesses of Bristol) in 1688. The town's common lands were enclosed by 1794, giving ownership mostly to the Herveys. This coincided with canalisation of the Slea, which brought economic growth until it was superseded by the railways in the mid-1850s. These new transport links supported the development of light industries and expanded the town's role in the trade in agricultural goods. Long a centre for justice and administration in north Kesteven, Sleaford became an urban district in 1894 and was home to Kesteven County Council's offices from 1925 to 1974. After a period of stagnation, in the late 20th and early 21st centuries the sale of farmland around Sleaford led to the development of large housing estates, causing the population to rapidly expand and the urban area to engulf Quarrington and Holdingham.

Though its traditional market has declined in the 21st century (and its cattle and corn markets shut in the 20th century) and much of its heavier manufacturing has departed, Sleaford's economy has diversified. The town remains an important administrative, service and commercial centre for the surrounding district. It houses supermarkets, shops and a large business park with offices and light manufacturing; the headquarters of North Kesteven District Council; three secondary schools (two of which are selective); four primary schools; three newspapers; police, fire and ambulance stations; several places of worship; many sports clubs; a leisure centre; and several medical and dental practices and care homes. Regeneration has transformed some earlier industrial areas, including through the construction of The Hub. The town is one of the largest employment centres in the district; the commonest employers in 2021 were the public sector, retail and, to a much lesser degree, manufacturing.

Sleaford is a civil parish and market town in the North Kesteven district of Lincolnshire. It is bounded by the civil parishes of Leasingham to the north; Ewerby and Evedon, and Kirkby la Thorpe to the east; Silk Willoughby to the south; and Wilsford, South Rauceby and North Rauceby to the west. These neighbouring parishes are rural, comprising villages separated from Sleaford's urban area by fields, though Kirkby la Thorpe also includes the Milton Way housing estate on Sleaford's eastern fringe.

Sleaford's urban area includes the town centre, focused on the marketplace (fronted by St Denys' Church), where Eastgate, Northgate, Southgate and Westgate meet. Though some parts have been redeveloped in the 20th century, including the Riverside Shopping Precinct and Flaxwell House, the area follows a medieval street layout and is home to many of the town's oldest buildings; it is also the retail and commercial hub. Carre Street (running parallel to Southgate to the east), once home to industry and wharves, has been regenerated in the late 20th and early 21st centuries.

To the north-west of the centre, housing developments along Northgate (which becomes Lincoln Road north of the railway line), mostly built in the 20th and early 21st centuries, have brought the hamlet of Holdingham into Sleaford's urban area, which extends as far north as the A17 and A15 junction at Holdingham Roundabout. To the town's north-east, the built-up area has expanded along Eastgate, where 19th-century housing closer to the town centre gives way to modern business parks; the River Slea forms the southern boundary of these developments and, closer to the town, Lollycocks Field sits between one of the business parks, Eastgate and the Slea. South of the river, the town's urban area extends eastwards along Boston Road, which runs from Southgate to the A17 at Kirkby la Thorpe. Except for Boston Road Recreation Ground, the road is straddled by housing west of the railway; developments near the centre are mostly 18th- and 19th-century, while those around Old Place, at the Hoplands and south of Boston Road are mostly planned 20th- or 21st-century residential estates.

The Victorian train station can be found near the southernmost end of Southgate; Station Road includes some converted 19th-century warehouses. Mareham Lane heads south out of the town, past the vast disused Bass Maltings complex. Also forking off from Southgate are Grantham Road and London Road, which fan out in a south-west direction. They link Sleaford with Quarrington village, which has a historic core that has been merged into the town's urban area by modern housing developments. The earliest suburban housing at the base of Southgate appeared in the 19th century and was known as New Quarrington. Ribbon development along London and Grantham roads is mostly early-20th-century; much larger planned developments took place in the late 20th and 21st centuries at Quarrington Hill, Southfields and between the two roads. To the town centre's west is Westgate, medieval in origin but heavily developed with dense terraced housing in the 19th century; to its north is Westholme, parkland which houses a school; south of Westgate is West Banks and its adjoining streets, between the River Slea and the Nine-Foot Drain, an area heavily built up in the 19th century. South of Westbanks are the remains of Sleaford Castle.