The Party of Left Socialist-Revolutionaries-Internationalists (Russian: Партия левых социалистов-революционеров-интернационалистов, romanized: Partiya levykh sotsialistov-revolyutsionerov-internatsionalistov) was a revolutionary socialist political party formed during the Russian Revolution.

In 1917, the Socialist Revolutionary Party split between those who supported the Russian Provisional Government, established after the February Revolution and those who supported the Bolsheviks, who favoured the overthrow of the Provisional Government and the placing of political power in the hands of the Congress of Soviets. Those that continued to support the Provisional Government became known as the Right SRs while those who aligned with the Bolsheviks became known as the Left Socialist-Revolutionaries or Left SRs (Russian: левые эсеры, romanized: leviye esery). After the October Revolution, the Left SRs formed a coalition government with the Bolsheviks from November 1917 to March 1918, but resigned its position in government after the signing of the Treaty of Brest-Litovsk. The Central Committee of the Left SRs ultimately ordered the assassination of Wilhelm von Mirbach in an attempt to cause Russia to re-enter World War I and launched an ill-fated uprising against the Bolsheviks shortly after. Most members of the Left SRs were promptly arrested, though the majority that opposed the uprising were steadily released and allowed to retain their positions in the Soviets and bureaucracy. However, they were unable to reorganize the party, which gradually splintered into multiple pro-Bolshevik parties – all of which would merge with the Russian Communist Party (Bolsheviks) by 1921.

The Left SRs were significantly underrepresented in the Russian Constituent Assembly due to outdated voter rolls which did not acknowledge the split between the Right and Left SRs.

The left-wing faction of the Socialist Revolutionary Party began to form after the February Revolution, grouping the most radical elements of the party. The internal faction was highlighted in the First All-Russian Congress of Soviets of Workers' and Soldiers' Deputies in mid-May 1917 for its position close to that of the Bolsheviks, while the bulk of the party aligned with the Mensheviks. The left-wing socialist revolutionaries were especially strong in the Petrograd Soviet, where they opposed the continuation of the First World War – which had been defended by the centrist fraction of the party since mid-April. They were also strong in the Northern Region, Kazan, Kronstadt, Helsinki and Kharkiv.

Later they became the main current in important rural provinces of the Russian interior, places where the socialist revolutionaries enjoyed the favor of the population. At the third party congress in May, they were a large and important fraction, although it was not until the crisis of autumn and the October Revolution when its support extended to the entire country. During the summer of 1917, it was gaining strength among soldier committees, both inside the country and in the front.

Except for Mark Natanson, at the head of the faction was a series of young leaders, from exile (Boris Kamkov), from Siberia (Maria Spiridonova) or agitation activities among the population (Prosh Proshian). The SR leadership, on the contrary, had veteran and conservative representatives, who led the party into an alliance with the liberals. This led the party to share government power but, at the same time, jeopardized its support among the population. As the year progressed, the leadership of the SRs moved further and further away from the feelings of its followers and its base, which favoured the leftist current. The number of socialist-revolutionary organizations and committees that followed the leftist faction grew, a trend that was accentuated in the early autumn. In general, workers and soldiers agreed with the positions of the left-wing, the intelligentsia continued to support the SR party line, and the peasants and local branches were divided among them. The executive committee of the largest railway union, the Vikzhel, elected on 23 August, had a majority of Left SRs. During the congresses of the regional, national and provincial soviets held between August and November, it was the effective division of the Right SRs and the strength of those on the left that often allowed the approval of leftist motions.

The leftists declared themselves the only representatives of the party program, and proclaimed the socialist character of the revolution, demanded the end of collaboration with the bourgeoisie and the immediate socialization of land, first with their surrender to the land committees and then to the peasants themselves. They were also opposed to the continuation of the war, even if it involved signing a separate peace with the Central Powers. In industrial policy, they advocated the granting of various rights (union organizing, living wages, eight-hour days) and workers' control of factories and played a prevalent role in the factory committees. Internationalists in the party wanted the extension of the revolution to other countries. They also advocated the transfer of government power to the Soviets, convinced that the provisional government did not apply the reforms they deemed necessary.

After the failed Kornilov coup, the leftist current took control of the socialist-revolutionary organization in the capital, traditionally more radical than that of other localities. Their growth within the SRs led them to hope that it would come under their control, delaying a split.