Upon the adoption of the decision on the unification in a joint state and constitution of the Kingdom of Serbs, Croats and Slovenes on December 1, 1918, nearly at the very beginning of its existence the government of Stojan Protić adopted a decision on the determination of state symbols which represented the Yugoslav state in the next twenty years. On December 9/22, 1918 in Belgrade, the government concluded that the state flag of the Kingdom of Serbs, Croats and Slovenes would be a tricolor with three equal horizontal fields – blue on the top, white in the middle and red on the bottom. Article 2 of the St Vitus's Day Constitution of 1921 confirmed the colours of the state flag of the Kingdom of Serbs, Croats and Slovenes as blue, white and red, horizontally placed against the vertical flagpole. In addition to the state flag, there were also the tribal flags – Serbian: red, blue and white; Croatian: red, white and blue and Slovenian: white, blue and red.

After World War II, when the structure of the Yugoslav society changed after the introduction of the new state organisation, abolishment of the monarchy and declaration of the republic of Yugoslavia, the state symbols were also changed.
Under the FPRY Constitution of February 1, 1946, the state flag kept the colours of the flag of the Kingdom of Yugoslavia with the same order of colours – blue, white and red. The new revolutionary authorities, who ensured the alteration of the social system during the war, added to the centre of the flag a red five-pointed star, the symbol under which they fought against fascism in World War II. The ratio between the width and length of the flag was 1-2.
The flag had three equal horizontal fields. The five-pointed star had a golden or yellow fimbriation. The flag did not change until the secession of some Yugoslav republics in the 1990s. The flag kept the same colours, but the five-pointed star was removed from it.