Leinster House (Irish: Teach Laighean) is the seat of the Oireachtas, the parliament of Ireland. Originally, it was the ducal palace of the Dukes of Leinster.

Since 1922, it has been a complex of buildings which houses Oireachtas Éireann, its members and staff. The most recognisable part of the complex and the "public face" of Leinster House continues to be the former ducal palace at the core of the complex.

Leinster House was the former ducal residence in Dublin of the Duke of Leinster, and since 1922 has served as the parliament building of the Irish Free State, the predecessor of the modern Irish state, before which it functioned as the headquarters of the Royal Dublin Society. The society's famous Dublin Spring Show and Dublin Horse Show were held on its Leinster Lawn, facing Merrion Square. The building is the meeting place of Dáil Éireann and Seanad Éireann, the two houses of the Oireachtas, and as such the term 'Leinster House' has become a metonym for Irish political activities.

Ireland's parliament over the centuries had met in a number of locations, most notably in the Irish Houses of Parliament at College Green, next to Trinity College Dublin. Its medieval parliament consisted of two Houses, a House of Commons and a House of Lords. Ireland's senior peer, the Earl of Kildare, had a seat in the Lords. Like all the aristocrats of the period, for the duration of the Social Season and parliamentary sessions, he and his family resided in state in a Dublin residence.

From the late eighteenth century, Leinster House (then called Kildare House) was the Earl's official Dublin residence. When it was first built in 1745–48 by James FitzGerald, Earl of Kildare, it was located on the unfashionable and isolated south side of the city, far from the main locations of aristocratic residences, namely Rutland Square (now Parnell Square) and Mountjoy Square. The Earl predicted that others would follow; in succeeding decades Merrion Square and Fitzwilliam Square became the primary location of residences of the aristocracy, with many of their northside residences being sold (many subsequently deteriorating and ending up as slums). The building itself was designed by architect Richard Cassels while some of the later elements and interior were designed by Isaac Ware.

In the history of aristocratic residences in Dublin, no other mansion matched Kildare House for its sheer size or status. When the Earl was made the first Duke of Leinster in 1766, the family's Dublin residence was renamed Leinster House. Its first and second floors were used as the floor model for the White House by Irish architect James Hoban, while the house itself was used as a model for the original stone-cut White House exterior.

One famous member of the family who occasionally resided in Leinster House was Lord Edward FitzGerald, who became involved with Irish nationalism during the 1798 Rebellion, which cost him his life. With the passage of the Act of Union in 1800, Ireland ceased to have its own parliament. Without a House of Lords to attend, increasing numbers of aristocrats stopped coming to Dublin, selling off their Dublin residences, in many cases to buy residences in London, where the new united parliament met.

The 3rd Duke of Leinster sold Leinster House in 1815 to the Royal Dublin Society. In 1853 the Great Industrial Exhibition was hosted in its grounds. The Natural History Museum was built on the site in 1857. Around the same time, two new wings were added, to house the National Library of Ireland and the National Museum of Ireland.