HMP Shepton Mallet, sometimes known as Cornhill, is a former prison in Shepton Mallet, Somerset, England. When it closed in 2013, it had been the United Kingdom’s oldest operating prison. Before closure, Shepton Mallet was a category C lifer prison holding 189 prisoners. 

The prison was opened before 1625 but was already in poor repair by the end of the First English Civil War in 1646. It was expanded in 1790 but conditions were again criticised in a report of 1822 and further building work was undertaken in the 1820s and 1830s. This included the installation of a treadwheel for those sentenced to hard labour. In 1843 the number of cells was increased by adding a second storey to each wing. The prison was damaged during a fire in 1904. In 1930 the number of inmates had fallen and the prison was closed.

Following the outbreak of the Second World War in 1939, the prison was reopened as a military prison. It was initially used by the British Army and later by American forces who constructed a new execution block to hang condemned prisoners. It was also used for the storage of important historical documents from the Public Record Office in London, including Domesday Book. Following the war the prison continued as a military “glasshouse” until it was returned to civilian use in 1966.

The prison was decommissioned in 2013 and now serves as a tourist attraction, with guided tours and other activities.