Leinster Rugby (Irish: Rugbaí Laighean) is one of the four professional provincial club rugby union teams from the island of Ireland. They compete in the United Rugby Championship and the European Rugby Champions Cup. The team represents the Leinster Branch, one of the four provincial unions that together make up the Irish Rugby Football Union (IRFU), but is not restricted to players from that province.

Leinster play their home games primarily at the 18,500-capacity RDS Arena, 'the RDS' in Dublin. Larger fixtures are played in the 51,700-capacity Aviva Stadium when the capacity of the RDS is insufficient or during its 2024–2026 renovation. Occasional one-off games been moved to Croke Park, home of the Gaelic Athletic Association and by some distance the largest stadium in Ireland. Before moving to the RDS in 2005, Leinster's traditional home ground was Donnybrook Stadium, in Dublin 4. The province plays primarily in blue with white or yellow trim and the team crest features a harp within a rugby ball, the harp being an ancient symbol of the province found in and taken from the flag of Leinster, although the traditional colours of Leinster Rugby mean the design more resembles the flag of the president of Ireland or the coat of arms of Ireland.

Leinster turned professional along with its fellow Irish provinces in 1995 and has competed in the United Rugby Championship (formerly known as the Celtic League, Magners League and the Pro12 / Pro14) since it was founded in 2001. Before 1995, the Leinster Branch had previously competed in the annual Irish interprovincial championship.

During their professional history, Leinster have been the most successful team in Ireland or from the Celtic League and its successors. As of 2025, Leinster have won their domestic title on nine occasions, while topping the regular season table a further five times, and have won as of 2025 all four URC Irish Shields. In Europe, Leinster are second only to Toulouse with four European Champions Cups, one EPRC Challenge Cup and five runners-up placings in the Champions Cup.

Leinster enjoy historical rivalries with all three provincial unions, most notably with Munster, where some games between the teams have been watched by over 80,000 fans in Croke Park. In addition, Leinster have notable rivalries with two French club teams; erstwhile rivals Toulouse with whom they have jockeyed for the title of Europe's most decorated team, and La Rochelle, a frequent opponent in European competition who defeated Leinster in back to back European Champions Cup finals in 2022 and 2023 under the direction of long-time Munster player Ronan O'Gara. In more recent years a rivalry has also developed with the South African Bulls franchise, with the sides meeting twice in three seasons in the semi-final of the United Rugby Championship, before meeting in the final of the 2024-25 championship.

The Leinster Branch was inaugurated at a meeting on 31 October 1879. The meeting was held at Lawrence's premises at 63 Grafton Street and was largely attended. Although this was the formal founding of Leinster as we know it today, with the amalgamation of the Irish Football Union and the Northern Union, the Leinster provincial team had been active since 1879 – when the first interprovincial derby was played against Ulster. The Leinster and Ulster teams also made up the representative Irish team that competed against England in Ireland's first-ever international in 1875. Upon the founding of the union, Munster were also added to the fray in 1879, when their first provincial team was selected and first Munster players represented Ireland.

F. Kennedy (Wanderers) was elected first Hon. Secretary of the Branch and C.B. Crocker (Lansdowne) first Hon. Treasurer.

The function of the Branch was to organise the game of rugby football in the province. Every year five representatives would be selected to join the IRFU Committee. They would be known as the "Leinster Five" and would pick the Leinster representative teams.