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 civil 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 nonsocialist, conservative-led Senate, commonly called the "Whites" (valkoiset). The Reds were supported by the Russian Soviet Republic, while the Whites received military assistance from the German Empire.

In 1917, Finland was a possession of the Russian Empire. The February and October Revolutions in 1917 in Russia led to the empire's defeat in World War I and its collapse. The chaos of Russia induced the breakdown of the Finnish state and disintegration of Finnish society, which was already experiencing economic, social, and political divisions and whose political system was in an unstable phase of democratization and modernization. As Finland was culturally and nationally uniform, its subsequent civil war did not involve ethnic or religious divisions.

The disintegration of Russia led to Finland's declaration of independence on 6 December 1917. Although the majority of the Finns supported sovereignty, the declaration occurred in the context of a power vacuum that followed the collapse of Russia. These events deepened the rivalry for the leadership of the 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. In the end, the crisis of power and authority penetrated all levels of society, from local administration to the workplaces.

As there were no generally accepted police and army forces to keep order in Finland after March 1917, the left and the right began building security groups of their own. Two paramilitary forces emerged: the White Guards and Red Guards. An atmosphere of political violence, fear, and mistrust arose in Finland. Fighting broke out between the Reds and the Whites during January 1918, and quickly escalated. The fate of the Finns during 1917–1918 has been seen often in the history of minor nations separating from (disintegrating) major countries.

The Reds carried out a general offensive from mid-February to early March, which failed. The general offensive of the Whites began on 15 March 1918. A large number of Russian soldiers remained stationed in Finland at the beginning 1918, but the majority of these troops were unwilling to fight, and were withdrawn from Finland by the end of March. Soviet Russia's main support to the Reds was the supply of weapons. The White offensive was bolstered by the intervention of the Baltic Sea Division of the German army in southern Finland on 3 April. The battles of Tampere and Viipuri won by the Whites and the Battle of Helsinki won by the German troops were the decisive military actions of the war, which was won by the Whites.

In the aftermath of the 1917–18 crisis and civil war, Finland passed from Russian rule to the sphere of influence of the German Empire. The conservative Finnish 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 an independent, democratic republic.

The civil war remains the most controversial and emotionally charged event in the history of modern Finland, and there have even been disputes about how to designate it. Approximately 37,000 people died during the conflict, most of them off the battlefields. Three quarters of the dead were Red combatants and sympathizers. 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, rent 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 shift to the peaceful development of the Finnish society was facilitated by the outcome of World War I and by the pre-1918 cultural and national unity of the nation.