Barnwell Manor is a Grade II listed country estate near the village of Barnwell, about 2.5 miles (4.0 km) south of Oundle, in Northamptonshire, England. The historic former home of the Duke and Duchess of Gloucester, as of 2017 it was occupied by Windsor House Antiques.
In 1938 Prince Henry, Duke of Gloucester, the third son of King George V, bought the house and estate with the bulk of his legacy from the late king. The Duke's wife Alice was the daughter of John Montagu Douglas Scott, 7th Duke of Buccleuch, a descendant of the Montagu family.
It was announced in January 1995 that the Gloucesters would vacate the house, so that Princess Alice could move to Kensington Palace to be with her son, the present Duke of Gloucester.  As of 2017, Windsor House Antiques occupies the estate.
In 2013 the High Court disallowed an application by West Coast Energy to build a wind farm close to the manor's lodge. An appeal was subsequently dismissed. The appeal gained increased media attention because Justine Thornton, wife of the Labour leader Ed Miliband, was representing the appellant. The Duke of Gloucester had supported the proposal.
The house has four reception rooms, seven principal bedrooms, and six bathrooms. It is a 40-room Grade II eighteenth-century manor house, with origins dating to 1586. The estate now comprises 2,500 acres (10 km2) farmed by the present Duke of Gloucester, and the ruined Barnwell Castle, built c.1266 by Berenger le Moyne, who sold it to Ramsey Abbey in 1276. The abbey held the castle until 1536, when it passed to the king. The Elizabethan manor house became the principal residence, and the living quarters and all internal buildings of the castle were demolished in 1704.