Albert I (8 April 1875 – 17 February 1934) was King of the Belgians from 23 December 1909 until his death in 1934. Albert was born in Brussels as the fifth child and second son of Prince Philippe, Count of Flanders, and Princess Marie of Hohenzollern-Sigmaringen, Albert succeeded his uncle Leopold II to the Belgian throne in 1909. He married Elisabeth of Bavaria, with whom he had three children.

Albert ruled during an eventful period in the history of Belgium, which included the period of World War I (1914–1918), when most of Belgium was occupied by German forces. Other crucial events of his reign included the adoption of the Treaty of Versailles in June 1919, the ruling of the Belgian Congo as an overseas possession of Belgium along with the League of Nations mandate of Ruanda-Urundi, the reconstruction of Belgium following the war, and the first five years of the Great Depression (1929–1934).

Albert died in a mountaineering accident in eastern Belgium in 1934, at the age of 58, and he was succeeded by his son Leopold III (r. 1934–1951). He is popularly referred to as the Knight King (Dutch: Koning-Ridder, French: Roi-Chevalier) or Soldier King (Dutch: Koning-Soldaat, French: Roi-Soldat) in Belgium in reference to his role during World War I.