​Sleaford (historically known as New Sleaford) is a market town and civil parish in Lincolnshire, England. Since 1973, the parish boundaries have included Quarrington to the south-west, Holdingham to the north and Old Sleaford to the east – contiguous settlements and former civil parishes which, with New Sleaford, had formed an Urban District. The town is on the edge of the fertile Fenlands, about 11 miles (18 kilometres) north-east of Grantham, 16 mi (26 km) west of Boston, and 17 mi (27 km) south of Lincoln. With a population of 17,671 at the 2011 Census, the town is the largest settlement in the North Kesteven district. Bypassed by the A17 and the A15, it is connected to Lincoln, Newark, Peterborough and King's Lynn. 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 a tribal centre and home to a mint for the Corieltauvi in the 1st centuries BC and AD. Evidence of Roman and Anglo-Saxon settlement has been uncovered. In the medieval period, 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 declined.

From the 16th century, the landowners were the Carre family, who operated tight control over the town, and it grew little in the early modern period. The manor passed from the Carre family to the Hervey family by the marriage of Isabella Carre to John Hervey, 1st Earl of Bristol, in 1688. The town's common land and fields were legally enclosed by 1794, giving ownership mostly to the Hervey family; this coincided with the Slea's canalisation; the Sleaford Navigation brought economic growth until it was superseded by the railways in the mid-1850s. In the 20th century, the sale of farmland around Sleaford by Bristol Estates led to the development of large housing estates. The subsequent availability of affordable housing combined with the town's educational facilities and low crime rates made it an attractive destination for home-buyers. As a result, the town's population underwent the fastest growth of any town in the county in the 1990s.

Sleaford was primarily an agricultural town until the 20th century, supporting a cattle market, with seed companies, such as Hubbard and Phillips, and Sharpes International Seeds, being established in the late 19th century. The arrival of the railway made the town favourable for malting. Industry has declined, and in 2011 the most common occupations are in wholesale and retail trade, health and social care, public administration and defence and manufacturing. Regeneration of the town centre has led to the redevelopment of the old industrial areas, including the construction of the National Centre for Craft & Design on an old wharf.