The Left Opposition was a faction within the Communist Party of the Soviet Union from 1923 to 1927 headed de facto by Leon Trotsky. The Left Opposition formed as part of the power struggle within the party leadership that began with the Soviet founder Vladimir Lenin's illness and intensified with his death in January 1924 . Originally, the battle lines were drawn between Trotsky and his supporters who signed The Declaration of 46 in October 1923, on the one hand, and a triumvirate (known by its Russian name troika) of Comintern chairman Grigory Zinoviev, Communist Party General Secretary Joseph Stalin and Politburo chairman Lev Kamenev on the other hand. The troika was supported by the leading party theoretician and Pravda editor Nikolai Bukharin and by Sovnarkom Chairman (prime minister) Alexei Rykov, who would later be branded the Right Opposition by Stalin. Trotsky and his supporters were joined by the Group of Democratic Centralism.

The first confrontation between the Left Opposition and the troika occurred in October 1923 - January 1924, first secretly and then, from early December on, openly. The troika won decisively at the XIIIth Party Conference in January 1924 and its victory was reaffirmed at the XIIIth Party Congress in June 1924. The second confrontation took place in October-December 1924 during the so-called "Literary Discussion" and ended with a removal of Leon Trotsky from his ministerial post on January 6, 1925.

With Trotsky largely marginalized, Kamenev and Zinoviev had a falling out with Stalin in early 1925. They formed the New Opposition, but were defeated by Stalin, who was again supported by Bukharin and Rykov, at the XIVth Party Congress in December 1925. After their defeat, Zinoviev and Kamenev joined forces with Trotsky's Left Opposition in what became known as the United Opposition. In July-October 1926 the United Opposition lost out to Stalin, and its leaders were expelled from the ruling Politburo. In October 1927, the last Opposition members were expelled from the Communist Party Central Committee, and in November 1927 Trotsky and Zinoviev were expelled from the Party itself. In December 1927, the XVth Party Congress declared Left Opposition and Trotskyist views to be incompatible with Party membership and expelled all leading oppositionists from the Party.

After the expulsion by the XVth Congress, Zinoviev, Kamenev and their supporters surrendered to Stalin, "admitted their mistakes" and were readmitted to the Communist Party in 1928, although they never regained their former influence and eventually perished in the Great Purge. Trotsky and his supporters, on the other hand, refused to bow to Stalin and were exiled to remote areas of the Soviet Union in early 1928. Trotsky was eventually expelled from the country in February 1929. His supporters remained in exile, but their resolve began to waver in 1929 as Stalin turned against Bukharin and Rykov and adopted the policy of collectivization, which appeared to be close to the policies that the Left Opposition advocated earlier. Most (but not all) prominent Left Opposition members recanted between 1929 and 1934, but perished during the Great Purge of the mid-late 1930s along with the oppositionists who remained unrepentant.

In the meantime, Trotsky founded the International Left Opposition in 1930. It was meant to be an opposition group within the Comintern, but members of the Comintern were immediately expelled as soon as they joined (or were suspected of joining) the ILO. The ILO therefore concluded that opposing Stalinism from within the Communist organizations controlled by Stalin's supporters had become impossible, so new organizations had to be formed. In 1933, the ILO was renamed the International Communist League (ICL), which formed the basis of the Fourth International, founded in Paris in 1938.

See the article on the Fourth International for the history of the Left Opposition after 1933.