The article concerns the little-studied aspects of the biography of the prominent political figure, literary critic, publicist and prose writer Yakov Veniaminovich Braun in order to reconstruct in its entirety the social and political activity of this multifaceted personality. The author examines the unrealized literary projects connected with the left-wing Socialist Revolutionary movement and repressions against Braun. The study is based on the archival and investigative materials, publications in newspapers and magazines, documents of the Russian State Archive of Socio-Political History and the Russian State Archive of Literature and Art. Most of the sources are introduced into scientific discourse. Throughout 1917–1933, Braun was an active participant in the Socialist Revolutionary and Left Socialist Revolutionary movements, member of the All-Ukrainian Committee of the Socialist Revolutionary Party in 1918–1919, secretary of the Party of Left Socialist Revolutionaries (syndicalists) of Ukraine in 1922–1923, member of the Central Committee of the United Left Socialist Revolutionary Party of Ukraine in 1922–1923 and the Central Bureau of the Association of Left Narodnik Movement (Association of the Party of Left Socialist Revolutionaries and the Union of Socialist Revolutionary Maximalists). He participated and acted as a speaker at party congresses and conferences. During his imprisonment and exile (with interruptions) in 1923–1933, Braun followed a left Narodnik direction and maintained contacts with like-minded people. However, breaking with them did not save him from arrest and execution in 1937. Braun’s activities as a party publicist can be traced through his articles in the newspapers Narodnoe Delo and Bor’ba and magazines Klich, Znamya and Narodnoe Prosveshchenie, as well as in the collection of articles Puti revolutsii. Also of interest are the unrealized literary projects of the publishing house “Revolutionnoye Tvorchestvo” and the magazine of the same name with the participation of Ivanov-Razumnik and Sergei Esenin. Particularly noteworthy is the letter from Braun to I.Z. Steinberg with a mention of his lecture titled Esenin and Mayakovsky in Kharkov in the spring of 1922 and a letter to Vladimir Mayakovsky dated November 8, 1927, in which Braun narrates about the book Esenin and Mayakovsky that he planned to write.