Andrei Sergeyevich Bubnov (Russian: Андрей Сергеевич Бубнов; 3 April [O.S. 22 March] 1883 – 1 August 1938) was a Russian Bolshevik revolutionary leader, Soviet politician and military leader, member of the Left Opposition, and an important Bolshevik figure in Ukraine.

Bubnov was born in Ivanovo-Voznesensk in Vladimir Governorate (now Ivanovo, Ivanovo Oblast, Russia) into a local Russian merchant's family. He studied at the Moscow Agricultural Institute, where he was involved in revolutionary circles beginning in 1900. He failed to graduate from the institute. In 1903, he joined the Bolshevik wing of the Russian Social Democratic Labour Party (RSDLP). In summer 1905, he joined the regional party committee of Ivanono-Voznesensk, and was their delegate to the 4th (1906) and 5th (1907) Party Conferences in Stockholm and London. Between 1907 and 1908, he was a member of the RSDLP's Moscow committee, and of the Bolshevik committee for the Central Industrial Region. He was arrested in 1908. In his autobiography, he stated that he was arrested 13 times during his revolutionary career, and spent four years imprisoned for his political activities.

On his release from prison in 1909 Bubnov was made an agent of the Central Committee in Moscow. He was arrested again in 1910, and interned in a fortress. After his release in 1911, he was sent to organize workers in Nizhny Novgorod. From there, he was one of the organisers of the Prague Conference of January 1912, the first that excluded all RSDLP members who were not Bolsheviks. He was under arrest at the time of the conference, but in his absence was elected a candidate member of the first all-Bolshevik Central Committee. Afterwards he was sent to St Petersburg to assist in the launch of Pravda, and to work with the Bolshevik faction in the Fourth Duma. Arrested yet again, he was deported to Kharkov.

On the outbreak of the First World War Bubnov became involved in the anti-war movement in Kharkiv, whence he had been deported after being expelled from St Petersburg. Arrested in August 1914, he was deported to Poltava. He moved to Samara, where he was arrested in October 1916, and exiled to Siberia in February 1917, but while he was in transit, he heard news of the February Revolution, and made his way back to Moscow.

In Moscow, Bubnov was elected to the Moscow Soviet and, at the 6th Party Conference in July 1917, he was elected to the Bolshevik central committee, which would later become the Central Committee of the Communist Party of the Soviet Union. In August, he moved to Petrograd, where he was a central figure during the October Revolution. On 23 October, two weeks before the revolution, the Central Committee appointed a seven-man political council consisting of Lenin, Zinoviev, Kamenev, Trotsky, Stalin, Sokolnikov and Bubnov. This is sometimes regarded as the first Politburo, but Trotsky's recollection was that this group was "completely impractical", since Lenin and Zinoviev were in hiding, and Zinoviev and Kamenev opposed the planned revolution, and "never once assembled." Bubnov's real importance was as a member of the Military Revolutionary Committee. "It was this body rather than the party 'politburo' which made the military preparations for the revolution." Together, they directed the inner-workings of would become known as the October Revolution. His role was to supervise the seizure of the postal and telegraph systems. After the successful execution of the October Revolution, he was appointed Commissar for Railways, before being sent to Rostov-on-Don to organise resistance to the newly formed White Army of General Kaledin.

Bubnov clashed with Lenin for the first time when he opposed the decision to sign the Treaty of Brest-Litovsk that ended the war with Germany. For the next four years, Bubnov was prominent in the left wing opposition to Lenin. He was dropped from the Central Committee in March 1918, but reinstated as a candidate member a year later. In February 1918, he joined the Left Communists, and moved to Ukraine, to organise partisan detachments in the 'neutral zone' east of the German front line. He and Georgy Pyatakov, who led the left in the Ukrainian party, argued that Ukraine was not a signatory to the Brest-Litovsk Treaty, and that they were therefore entitled to organise partisan war against the Germans.

In October 1918, Bubnov moved to Kyiv, which was ruled by Hetman Skoropadskiy, with German backing, and later by the Ukrainian nationalist Symon Petliura. Bubnov acted as the chairman of the clandestine Kyiv soviet, retaining that position after the Red Army had taken Kyiv. During the Russian Civil War, he was a political commissar with the Red Army, fighting on the Ukrainian Front. During the Ninth Party Congress in Moscow, in March 1920, he accused the central party leadership of wrecking the party organisation in Ukraine by removing oppositionists, and threatening the stability of the Ukraine government by alienating peasant farmers. He was again removed from the Central Committee and, soon afterwards, he was recalled to Moscow to take charge of the textile industry. At the next party congress, in March 1921, he acted as a spokesman for the "Democratic Centralists", who demanded less centralised control of the communist party, but on hearing of the outbreak of the Kronstadt rebellion, rushed north to take part in suppressing, for which he was awarded the Order of the Red Banner.

In 1921–22, Bubnov was posted in the North Caucasus. In 1922, he was appointed head of the Agitprop department of the Central Committee, which meant he was working alongside Stalin, the new General Secretary.