Marien Ngouabi (or N'Gouabi; 1938 - March 18, 1977) was the military President of the Republic of the Congo from January 1 1969 - March 18 1977.

As a Captain he and other officers had overthrown the government of Alphonse Massamba-Débat in 1968 as part of a lefist coup d'etat. After a period under the National Revolutionary Council with Ngouabi as chairman and Alfred Raoul as acting head of state, Ngouabi assumed the presidency on December 31, 1968. On December 31, 1969 President Ngouabi changed the country's name to the People's Republic of the Congo, declaring it to be Africa's first Marxist-Leninist state and announcing that the National Revolutionary Movement (MNR) would become the Congolese Workers' Party (Parti Congolais du Travail, PCT), the country's sole legal political party.

Ngouabi was from the north (he was a Kouyou , born in the village of Ombele) and his regime shifted control of the country away from the south. Such moves created opposition among the population in the highly politicized environment of Brazzaville. There was an attemped coup in 1972 that triggered a series of 'purges' of the opposition. It is claimed that Ngouabi was under French pressure to annex the oil-rich Cabinda enclave then under Angolan control and his refusal to act cost him French support. There is some speculation that the French financed some of the following attempts to remove Ngouabi. He visited the People's Republic of China in July 1973. He was 'reelected' in 1975 and in the same year, he signed an economic aid pact with the Soviet Union.

On March 18, 1977, President Ngouabi was assassinated by an alleged suicide commando. The persons accused of taking part in the plot were tried and some of them executed including Massamba-Débat. An Military Committee of the Party (CMP) was named to head an interim government with the conservative Colonel Joachim Yhombi-Opango to serve as President of the Republic.

March 18 is Marien Ngouabi Day in the Republic of Congo. The country's only university is the Université Marien Ngouabi in Brazzaville.