The Williamite War in Ireland (1688–1691; Irish: Cogadh an Dá Rí, "war of the two kings"), was a conflict between Jacobite supporters of deposed monarch James II and Williamite supporters of his successor, William III. It is also called the Jacobite War in Ireland, Williamite Conquest of Ireland, or the Williamite–Jacobite War in Ireland.

The proximate cause of the war was the Glorious Revolution of 1688, in which James, a Catholic, was overthrown as king of England, Ireland and Scotland and replaced by his Protestant daughter Mary and nephew and son-in-law William, ruling as joint monarchs. James's supporters initially retained control of Ireland, which he hoped to use as a base for a campaign to reclaim all three kingdoms. The conflict in Ireland also involved long-standing domestic issues of land ownership, religion and civic rights; most Irish Catholics supported James in the hope he would address their grievances. A small number of English and Scottish Catholics, and Protestants of the Anglican established Church in Ireland, also fought on the Jacobite side, while most Irish Protestants supported or actively fought for William's regime.

While the war's Irish name emphasises its aspect as a domestic conflict between James and William, some contemporaries and many modern commentators have viewed it as part of a wider European conflict known as the Nine Years' War or War of the Grand Alliance in which William, as Stadtholder of the Dutch Republic, led a multi-national coalition against France under Louis XIV. William's deposition of James was partly driven by his need to control and mobilise English military and commercial power, while Louis provided limited material support to the Jacobites: both sides were aware of the Irish war's potential to divert military resources from the Continent.

The war began with a series of skirmishes between James's Irish Army, which had stayed loyal in 1688, and militia forces raised by Irish Protestants: they culminated in the siege of Derry, where the Jacobites failed to regain control of one of the north's key towns. William landed a force including English, Scottish, Dutch, Danish and other troops to put down Jacobite resistance. James left Ireland after a reverse at the Battle of the Boyne in 1690, as did William after a successful Jacobite defence of Limerick; the remaining Jacobite forces were decisively defeated at the Battle of Aughrim in 1691, and negotiated terms in the Treaty of Limerick.

A contemporary witness, George Story, calculated that the war had claimed 100,000 lives through sickness, famine, and in battle. Subsequent Jacobite risings were confined to Scotland and England, but the war was to have a lasting effect on the political and cultural landscape of Ireland, confirming British and Protestant rule over the country for over two centuries. While the Treaty of Limerick had offered a series of guarantees to Catholics, subsequent extension of the Penal Laws, particularly during the War of the Spanish Succession, would further erode their civic rights.

The Williamite victories at Derry and the Boyne are still celebrated by some, mostly Ulster Protestant, unionists in Ireland today.