Watford Gap /ˈwɒtfərd ɡæp/ is a low-lying area between two hills, close to the village of Watford, Northamptonshire, England. Engineers from Roman times onwards have found it to be an ideal route for connecting the Midlands with South East England. The A5 road, the West Coast Main Line railway, the M1 motorway and a branch of the Grand Union Canal traverse in parallel a space about 400 metres (1,300 ft) wide. It has been written and spoken of as marking the divide between Northern England and Southern England.

Watford Gap is a low point through a range of hills providing an easy route between the South Eastern and Midland areas of England. It is near the small village of Watford, Northamptonshire. The gap is 3.5 miles (6 km) north-east of Daventry and 2 miles (3 km) west of Long Buckby. In the era of Roman Britain, the Watling Street Roman road used the gap. The road here forms the A5, which for national journeys has generally been superseded by the M1 motorway which also passes through this gap.

Later the road was joined by the Leicester Line of the Grand Union Canal, which passes through the Watford Locks just to the north of the gap.

The historical geographic importance of the area led to many modern communication routes passing through this narrow gap: the coming of the railways brought the London and Birmingham Railway, now known as the West Coast Main Line; the most recent addition, in 1959, was the M1, Britain's first inter-urban motorway, bringing with it Watford Gap services, the first motorway service station.

The village is near to Watford Gap services on the M1 motorway, which was the first motorway service station in the United Kingdom.

Roy Harper's 1977 album Bullinamingvase contains a song titled "Watford Gap". Motorway service areas, which have since undergone radical innovations, were at the time in the United Kingdom typecast as spartan. The owners of Watford Gap services objected to his first reference to the place being ("Watford Gap, Watford Gap / A plate of grease and a load of crap…"). Harper was advised^{[by whom?]} to drop the track from future UK copies of the album, though it reappeared on a CD reissue and remained on the American LP.^{[citation needed]}