Northamptonshire (/nɔːrˈθæmptənʃər, -ʃɪər/ nor-THAMP-tən-shər, -⁠sheer; abbreviated Northants.) is a ceremonial county in the East Midlands of England. It is bordered by Leicestershire, Rutland and Lincolnshire to the north; Cambridgeshire to the east; Bedfordshire and Buckinghamshire to the south; Oxfordshire to the south west; and Warwickshire to the west. Northampton is the largest settlement and the county town.

The county has an area of 2,364 km² (913 sq mi) and a population of 747,622. The latter is concentrated in the centre of the county, which contains the county's largest towns: Northampton, Corby, Kettering, and Wellingborough. The northeast and southwest are rural. The county contains two local government districts, North Northamptonshire and West Northamptonshire, which are both unitary authority areas. The historic county included the Soke of Peterborough.

The county is characterised by low, undulating hills, particularly to the west. They are the source of several rivers, including the Avon and Welland, which form much of the northern border; the Cherwell; and the Great Ouse. The River Nene is the principal river within the county, having its source in the southwest and flowing northeast past Northampton and Wellingborough. The highest point is Arbury Hill southwest of Daventry, at 225 m (738 ft).

There are Iron Age and Roman remains in the county, and in the seventh century it was settled by the Angles and Saxons, becoming part of Mercia. The county likely has its origin in the Danelaw as the area controlled from Northampton, which was one of the Five Boroughs. In the later Middle Ages and Early Modern Period the county was relatively settled, although Northampton was the location of engagements, including the 1264 Battle of Northampton during the Second Barons' War, the 1460 Battle of Northampton during the Wars of the Roses, and the decisive Battle of Naseby which destroyed the main Royalist army during the First English Civil War. During the Industrial Revolution Northamptonshire became known for its footwear, and the contemporary county has a number of small industrial centres which specialise in engineering and food processing.

Much of Northamptonshire's countryside appears to have remained somewhat intractable as regards early human occupation, resulting in an apparently sparse population and relatively few finds from the Palaeolithic, Mesolithic and Neolithic periods. In about 500 BC the Iron Age was introduced into the area by a continental people in the form of the Hallstatt culture, and over the next century a series of hill-forts were constructed at Arbury Banks (Arbury Camp), Rainsborough camp, Borough Hill, Castle Dykes, Guilsborough, Irthlingborough, and most notably of all, Hunsbury Hill. There are two more possible hill-forts at Arbury Hill (Badby) and Thenford. In the 1st century BC, most of what later became Northamptonshire became part of the territory of the Catuvellauni, a Belgic tribe, the Northamptonshire area forming their most northerly possession. The Catuvellauni were conquered by the Romans in 43 AD.

The Roman road of Watling Street passed through the county, and an important Roman settlement, Lactodurum, stood on the site of modern-day Towcester. There were other Roman settlements at Northampton, Kettering and along the Nene Valley near Raunds. A large fort was built at Longthorpe.

After the Romans left, the area eventually became part of the Anglo-Saxon kingdom of Mercia, and Northampton functioned as an administrative centre. The Mercians converted to Christianity in 654 AD with the death of the pagan king Penda. From about 889 the area was conquered by the Danes (as at one point almost all of England was, except for Athelney marsh in Somerset) and became part of the Danelaw – with Watling Street serving as the boundary – until being recaptured by the English under the Wessex king Edward the Elder, son of Alfred the Great, in 917. Northamptonshire was conquered again in 940, this time by the Vikings of York, who devastated the area, only for the county to be retaken by the English in 942.

The county was first recorded in the Anglo-Saxon Chronicle (1011), as Hamtunscire: the scire (shire) of Hamtun (the homestead). The "North" was added to distinguish Northampton from the other important Hamtun further south: Southampton – though the origins of the two names are in fact different.