Bubnov was born in Ivanovo-Voznesensk in Vladimir Governorate (now Ivanovo, Ivanovo Oblast, Russia) on 23 March 1883 into a local Russian merchant's family. He was expelled from Moscow University for revolutionary activities. He studied at the Moscow Agricultural Institute and while a student joined the Russian Social Democratic Labour Party (RSDLP) in 1903. He was a supporter of the Bolshevik faction of the party. In summer 1905, he joined the Ivanono-Voznesensk party committee, and was their delegate to the 4th (1906) and 5th (1907) Party Conferences in Stockholm and London. In 1907–08, 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.