Harmondsworth Great Barn

Harmondsworth Great Barn is a medieval barn on the former Manor Farm in the village of Harmondsworth, in the London Borough of Hillingdon, England. It is north-west of fields and the A4 next to Heathrow Airport. Built in the early 15th century by Winchester College, it is the largest timber-framed building in England and is regarded as an outstanding example of medieval carpentry. It was described by the English poet John Betjeman as the "Cathedral of Middlesex". A similar though smaller barn is part of the Manor Farm complex in Ruislip.

The barn was briefly in royal ownership but passed into the hands of three families who continued to use it for agricultural purposes until as late as the 1970s. It was subsequently owned by a property development company which redeveloped the farm complex.