Holdenby House is a historic country house in Northamptonshire, traditionally pronounced, and sometimes spelt, Holmby. The house is situated in the parish of Holdenby, six miles (10 km) northwest of Northampton and close to Althorp. It is a Grade II* listed building.
The house was completed in 1583 by the Elizabethan Lord Chancellor, Sir Christopher Hatton, who refused to sleep a night in the mansion until Queen Elizabeth I had slept there. It was one of the largest prodigy houses of the Tudor period, rivalling in size both Audley End and Theobalds, and was reputed to occupy approximately 78,750 square feet (7,300 m²), although this probably included the two great courtyards around which it was built. The facades were symmetrical, with mullioned windows and open Doric arcades, reflecting the renaissance style of architecture gradually spreading from Italy. The cost of building Holdenby bankrupted Hatton, who died in 1591.
In 1607 the mansion was bought by Elizabeth's successor James I. In February 1647, after the First English Civil War, Charles I was brought to Holdenby by the Scots and handed over to the English Long Parliament. He remained a prisoner there until June 1647 when Cornet George Joyce seized him and took him to Newmarket in the name of the New Model Army. Parliament later sold the estate to Captain Adam Baynes, who demolished the house almost entirely except for a small domestic wing.