Nelson's Pillar (also known as the Nelson Pillar or simply the Pillar) was a large granite column capped by a statue of Horatio Nelson, built in the centre of what was then Sackville Street (later renamed O'Connell Street) in Dublin, Ireland. Completed in 1809 when Ireland was part of the United Kingdom, it survived until March 1966, when it was severely damaged by explosives planted by Irish republicans. Its remnants were later destroyed by the Irish Army.

The decision to build the monument was taken by Dublin Corporation in the euphoria following Nelson's victory at the Battle of Trafalgar in 1805. The original design by William Wilkins was greatly modified by Francis Johnston, on grounds of cost. The statue was sculpted by Thomas Kirk. From its opening on 29 October 1809 the Pillar was a popular tourist attraction, but provoked aesthetic and political controversy from the outset. A prominent city centre monument honouring an Englishman rankled as Irish nationalist sentiment grew, and throughout the 19th century there were calls for it to be removed, or replaced with a memorial to an Irish hero.

It remained in the city as most of Ireland became the Irish Free State in 1922, and the Republic of Ireland in 1949. The chief legal barrier to its removal was the trust created at the Pillar's inception, the terms of which gave the trustees a duty in perpetuity to preserve the monument. Successive Irish governments failed to deliver legislation overriding the trust. Although influential literary figures such as W. B. Yeats and Oliver St. John Gogarty defended the Pillar on historical and cultural grounds, pressure for its removal intensified in the years preceding the 50th anniversary of the Rising, and its sudden demise was, on the whole, well-received by the public. Although it was widely believed that the action was the work of the Irish Republican Army (IRA), the police were unable to identify any of those responsible.

After years of debate and numerous proposals, the site was occupied in 2003 by the Spire of Dublin, a slim needle-like structure rising almost three times the height of the Pillar. In 2000, a former republican activist gave a radio interview in which he admitted planting the explosives in 1966, but, after questioning him, the Gardaí decided not to take action. Relics of the Pillar are found in Dublin museums and appear as decorative stonework elsewhere and its memory is preserved in numerous works of Irish literature.