Sir Robert David Muldoon (; 25 September 1921 – 5 August 1992) was a New Zealand politician who served as the 31st prime minister of New Zealand, from 1975 to 1984, while leader of the National Party.

Serving as a corporal and sergeant in the army in the Second World War, Muldoon completed his training as an accountant and returned to New Zealand as its first fully qualified cost accountant. He was first elected to the House of Representatives at the 1960 general election as the Member of Parliament (MP) for Tamaki, representing the National Party. In this time of political stability, Muldoon served successively as Minister of Tourism (1967), Minister of Finance (1967–1972), and Deputy Prime Minister (1972). Over this time he built up an informal but solid backing amongst National's mostly rural right faction, which he labelled "Rob's Mob"—possibly in imitation of gangs such as the Mongrel Mob. National were then expelled from office in 1972, beginning the tenure of Labour Party Prime Minister Norman Kirk and ending 12 years of the Second National Government. Using his newfound connections, Muldoon was able to oust moderate party leader Jack Marshall and take his place, becoming Leader of the Opposition in 1974. Through his own mixture of moderate liberalism and protectionist right-wing populism ("counterpunching", a term he coined), and promise of a lucrative superannuation scheme, Muldoon gained widespread support quickly. The early death of Norman Kirk severely weakened the Labour Party, and Muldoon soon led the National Party to a decisive victory in the 1975 general election.

Muldoon came to power promising to lead "a Government of the ordinary bloke." He appointed himself Minister of Finance. Although he used populist rhetoric to rail against elites and the political establishment, he consistently tried to centralise power under himself during his premiership. His tenure was plagued by an economic pattern of stagnation, high inflation, growing unemployment, and high external debts and borrowing. Economic policies of the Muldoon Government included national superannuation, wage and price freezes, industrial incentives, and the Think Big industrial projects. He reintroduced and intensified the previous government's policies of the Dawn Raids, which racially targeted Pasifika overstayers. To engage with crime, Muldoon built "unusually close relationships" with criminal gangs; he personally favoured Black Power, and he and his wife Thea met with them on several occasions. In foreign policy, Muldoon adopted an anti-Soviet stance and re-emphasised New Zealand's defence commitments to the United States and Australia under the ANZUS pact. His refusal to stop a Springbok rugby tour of New Zealand divided the country and led to unprecedented civil disorder in 1981. Muldoon became more and more controversial as his premiership progressed; in addition to the controversy of the Springbok tour, he began a smear campaign against Labour MP Colin Moyle for allegedly being gay, visited strip clubs, and once personally punched demonstrators at a protest.

Muldoon led his party to two additional election victories in 1978 and 1981, with the first-past-the-post electoral system keeping him in power despite losing the popular vote in every election except 1975. At the 1984 snap election, which Muldoon infamously called while visibly drunk, National finally suffered a significant defeat to Labour. Shortly before leaving office, amid a constitutional crisis, Muldoon was forced by the incoming Government to devalue the New Zealand dollar. In 1984, he was only the second Prime Minister (after Sir Keith Holyoake) to receive a knighthood while still in office. He died in 1992; having gained their respect by engaging with them directly, the gang Black Power performed a haka at his funeral.

Departing significantly from National Party convention, Muldoon had a controversial public image. He has been called the first world leader to be so openly informal and abrasive. Described as having "run the country like a mob boss", he was willing to enact tougher and harsher policies than previous Prime Ministers. His legacy is one of recognition as being among the most preeminent Western statesmen of his generation, and an "enigma" who openly broke social norms and bullied his peers. In the 21st century, Muldoon has been recognised for having laid the footing for the likes of strongman-like leaders such as Donald Trump and Silvio Berlusconi.