The Clerkenwell explosion, also known as the Clerkenwell Outrage, was a bombing attack carried out by the Irish Republican Brotherhood (IRB) in London on 13 December 1867. Members of the IRB, who were nicknamed "Fenians", exploded a bomb to try to free a member of their group who was being held on remand at Clerkenwell Prison. The explosion damaged nearby houses, killed 12 civilians and wounded 120; no prisoners escaped and the attack was a failure. The event was described by The Times the following day as "a crime of unexampled atrocity", and compared to the "infernal machines" used in Paris in 1800 and 1835 and the Gunpowder Treason of 1605. Denounced by politicians and writers from both sides of the political spectrum, the bombing was later described as the most infamous action perpetrated by Fenians in Britain during the 19th century. It enraged the British public, causing a backlash which undermined the Irish Home Rule Movement.

The Irish Republican Brotherhood was a secret oath-bound fraternal organisation dedicated to the establishment of an "independent democratic republic" in Ireland between 1858 and 1924. Its counterpart in the United States of America was initially the Fenian Brotherhood, but from the 1870s it was Clan na Gael. The members of both wings of the movement are often referred to as "Fenians". The IRB played an important role in the history of Ireland, as the chief advocate of republicanism during the campaign for Ireland's independence from the United Kingdom, successor to movements such as the United Irishmen of the 1790s and the Young Irelanders of the 1840s.