The River Witham is a river almost entirely in the county of Lincolnshire in the east of England. It rises south of Grantham close to South Witham at SK8818, passes through the centre of Grantham, passes Lincoln at SK9771 and at Boston, TF3244, flows into The Haven, a tidal arm of The Wash, near RSPB Frampton Marsh. The name "Witham" seems to be extremely old and of unknown origin. Archaeological and documentary evidence shows the importance of the Witham as a navigable river from the Iron Age onwards. From Roman times it was navigable to Lincoln, from where the Fossdyke was constructed to link it to the River Trent. The mouth of the river moved in 1014 following severe flooding, and Boston became important as a port.

Lincolnshire is a ceremonial county in the East Midlands and Yorkshire and the Humber regions of England. It is bordered by the East Riding of Yorkshire across the Humber estuary to the north, the North Sea to the east, Norfolk, Cambridgeshire, Northamptonshire and Rutland to the south, and Leicestershire, Nottinghamshire and South Yorkshire to the west. The county town is the city of Lincoln. Lincolnshire is the second largest county in England after North Yorkshire.