The Finnish Civil War (Finnish: Suomen sisällissota; Swedish: Finska inbördeskriget) was a part of the national and social turmoil caused by World War I (1914–1918) in Europe. The war was fought in Finland from 27 January to 15 May 1918, between the forces of the Social Democrats led by the People's Deputation of Finland, commonly called the "Reds" (punaiset), and the forces of the non-socialist, conservative-led Senate, commonly called the "Whites" (valkoiset). The Reds were supported by Russian Soviet Federative Socialist Republic, while the Whites received military assistance from the German Empire.

The February and October Revolutions in 1917 led to both the Russian Empire's defeat in World War I and its total collapse, including the northwestern territory of the Empire, the land of the Finns. The destruction in Russia resulted in a corresponding collapse of the Finnish administrative, economic and military control, as well as the breakdown of Finnish society during 1917. Contrary to many civil wars, the war of the Finns was not fought between ethnic or religious groups, instead the turmoil concerned a nation that was culturally and nationally rather uniform, but divided economically, socially and politically, with a political system under an unstable phase of democratization and modernization. The power vacuum that followed the collapse of Russia led to a struggle for the leadership of the Finnish state between the left-leaning labour movement (the Social Democrats) and the more right-wing conservatives. Both sides were reluctant to make political compromises and aimed to gain supremacy for their own faction, due to the nature of the political and social structure of the nation in the 19th century. In the end, the crisis of power and authority penetrated all levels of society, from local administration to workplaces. The disintegration of Russia led to Finland's declaration of independence on 6 December 1917 and the majority of the Finns supported sovereignty, but on the other hand it deepened the struggle for the leadership of the state further.

As there were no generally accepted police and army forces to keep order in Finland after March 1917, the left and right began building security groups of their own, leading to the emergence of two independent armed military troops, the White and Red Guards, and a stage of dual military and political power. An atmosphere of political violence, fear and mistrust grew among the Finns. Fighting broke out during January 1918 due to the acts of both the Reds and Whites in a spiral of military and political escalation. The faith of the Finns during 1917-1918 was in accordance with the concept often manifested in the history of minor nations separating from (disintegrating) major countries.

The Whites were victorious in the ensuing war. The Reds carried out a failed general offensive between half of February and beginning of March 1918. A high number of Russian soldiers remained stationed in Finland in the beginning 1918, but as the majority of the troops were unwilling to fight, and were withdrawn from Finland by the end of March, the main support of soviet-Russia to the Reds was supplying a marked number of weapons. The general offensive of the Whites was issued on 15 March 1918. The offensive was followed by the effective intervention of the Baltic Sea Division of the German army in southern Finland on 3 April. Battles of Tampere and Viipuri won by the Whites, and Battle of Helsinki won by the German troops were the decisive military actions of the war.

In the aftermath of the 1917–18 crisis and the Civil War, Finland passed from Russian rule to the German sphere of influence. The conservative senate attempted to establish a Finnish monarchy ruled by a German king, but after the defeat of Germany in World War I, Finland emerged as a factually independent, democratic republic.

The Civil War remains the most controversial and emotionally loaded event in the history of modern Finland, and there have even been disputes about what the conflict should be called. Approximately 37,000 people died during the conflict, 75 % of them were Finnish Reds. The majority of the casualties did not take place in the battles at the war fronts, instead most of the deaths were caused by political terror campaigns and high prison camp mortality rates. The turmoil caused a severe food shortage, destroyed the economy, split the political apparatus, and divided the Finnish nation for many years. The country was slowly reunited through the compromises of moderate political groups on the left and right. The primarily peaceful development of the Finnish society after 1918 was induced by the outcome of World War I, and the pre-1918 cultural and national integrity of the nation.