The State Flag of the Union of Soviet Socialist Republics, also simply known as the Soviet flag or the Red Banner, was a red flag with two communist symbols displayed in the canton: a gold hammer and sickle topped off by a red five-point star bordered in gold. The flag's design and symbolism are derived from several sources, but emerged during the Russian Revolution. It has also come to serve as the standard symbol representing communism as a whole, recognized as such in international circles, even after the dissolution of the Soviet Union in 1991.

The plain red flag, which was a traditional revolutionary symbol long before 1917, was incorporated into the Soviet flag to pay tribute to the international aspect of the workers' revolution. On the other hand, the unique hammer-and-sickle design was a modern industrial touch adopted from the Russian Revolution; it represented the "victorious and enduring revolutionary alliance" by unifying the hammer (i.e. workers) and the sickle (i.e. peasants). The gold-bordered five-point star situated above the hammer and sickle was a representation of the Communist Party of the Soviet Union.

The first flag was adopted in December 1922. In 1923, 1924, 1936, and in 1955, statutes were adopted that resulted in adjustments to the hammer's handle length and the sickle's shape. In 1980, an amendment was made to the 1955 decree that removed the hammer and sickle displayed on the flag's reverse side, though the legal description remained completely unchanged. The design of the 1955 Soviet flag served as the basis for all of the flags of the Union Republics.

The flag of the Soviet Union consisted of a plain red flag with a gold hammer crossed with a gold sickle placed beneath a gold-bordered red star. This symbol is in the upper left canton of the red flag.

The colour red honours the red flag of the Paris Commune of 1871; the red star and the hammer and sickle are symbols of communism and socialism.

The hammer symbolises urban industrial workers while the sickle symbolises agricultural workers (peasants), who form the state together with the proletarian class. The red star represents the Communist Party, and its position over the hammer and sickle symbolises its leading role in socialist society to unify and enlighten the workers and peasants in the building of a communist society.

The flag's design was legislated in 1955, which gave a clear way to define and create the flag. This resulted in a change of the hammer's handle length and the shape of the sickle. The adopted statute stated that:

Officially since 1980, the reverse side of the flag was a plain red field without the hammer and sickle. In practice, however, this was very commonly disregarded by flag makers as it was far easier and less costly to simply print the flag through and through, with the obverse design mirrored on the reverse. It was also common to see the reverse of the flag bear the hammer and sickle in the obverse formation. An example of the flag demonstrating its de jure status as being only one-sided is that of the Soviet flag atop the Moscow Kremlin which bore the single-side official design.