Harmondsworth Great Barn is a historic barn located in the village of Harmondsworth, near Heathrow Airport in England. Built in the late 14th century, it is one of the largest medieval barns in the country, measuring about 50 meters long. The barn was originally used for storing grain and other agricultural products. The structure is notable for its impressive timber frame, constructed using oak beams and featuring a thatched roof. Today, Harmondsworth Great Barn is a protected heritage site, showcasing the architectural skills of the medieval period and serving as a reminder of the region's agricultural history.