Dungannon (from Irish Dún Geanainn, meaning 'Geanann's fort', pronounced [d̪ˠuːn̪ˠ ˈɟan̪ˠən̪ˠ]) is a town in County Tyrone, Northern Ireland. It is the second-largest town in the county (after Omagh) and had a population of 16,282 at the 2021 Census. The Dungannon and South Tyrone Borough Council had its headquarters in the town, though since 2015 the area has been covered by Mid-Ulster District Council.

For centuries, it was the 'capital' of the O'Neill dynasty of Tír Eoghain, who dominated most of Ulster and built a castle on the hill. After the O'Neills' defeat in the Nine Years' War, the English founded a plantation town on the site, which grew into what is now Dungannon. A linen centre in the 19th century, it became a centre for food processing in the late 20th, with Moy Park, a leading poultry producer, today its largest employer. As a result of the processors sourcing immigrant labour, Dungannon currently has the highest percentage of residents born outside of the British Isles of any town in Northern Ireland.

For centuries, Dungannon's fortunes were closely tied to that of the O'Neill dynasty which ruled a large part of Ulster until the 17th century. Dungannon was the clan's main stronghold. The traditional site of inauguration for 'The O'Neill' was Tullyhogue Fort, an Iron Age mound some four miles northeast of Dungannon. The clan O'Hagan were the stewards of this site for the O'Neills. In the 14th century the O'Neills built a castle on what is today known as Castle Hill; the location was ideal for a fort, for it was one of the highest points in the area and dominated the surrounding countryside, giving (depending on the weather) the ability to see seven counties.

This castle was burned in 1602 by Hugh O'Neill, 2nd Earl of Tyrone, as Crown forces under Lord Mountjoy closed in on the Gaelic lords towards the end of the Nine Years' War. In 1607, ninety-nine Irish chieftains and their followers, including Hugh O'Neill, set sail from Rathmullan, bound for the continent, in an event known as the Flight of the Earls. In what became known as the Plantation of Ulster, their lands were confiscated and awarded to Protestant English and Scots settlers; Dungannon and its castle were granted to Sir Arthur Chichester, the Lord Deputy of Ireland.

As part of the Plantation, in 1608 James I chartered a number of '"free schools" for the sons of local merchants and farmers. This included the Royal School Dungannon, established in the town in 1636, and occupying its present site south-east of Castle Hill from 1789 with the erection of the building we now know as the "Old Grey Mother" by the Archbishop of Armagh, Richard Robinson.

Sir Phelim O'Neill seized the town in the opening stages of the Irish Rebellion of 1641, and issued the Proclamation of Dungannon, in which the rebels set out their aims and proclaimed their loyalty to Charles I. O'Neill claimed they had been ordered to rise by the King, and later produced a forged commission in support of this.

During the course of the Irish Confederate Wars, Dungannon changed hands several times; Scots Covenanter forces under Alexander Leslie captured it in September 1642, before O'Neill took it back in spring 1643.

In 1689, during the Williamite War, Castle Hill, with still extant fortifications, was occupied by a Jacobite force, and hosted King James II as he passed en route to the Siege of Derry. In 2007, the castle was partially excavated by the Channel 4 archaeological show Time Team, uncovering part of the moat and walls of the castle.