This article offers biographical sketches of the Jewish members of the Bolshevik revolutionary élite. It explores how their commitments to socialist universalism and eventual identification with Bolshevism were influenced by experiences and identities as Jews in fin de siècle Tsarist Russia. Situating them within a comparative historical sociology of ethnicity and identity across the Empire, I consider the ways in which ambiguities of assimilation, ethnic exclusion, and ethnocultural marginality influenced their attraction to Bolshevik socialism. In doing so, I revise the traditional argument that that the Bolsheviks of Jewish origin were highly assimilated “non-Jewish Jews” whose Jewishness played no role in their political radicalism. Instead, the claim is made that for the Jewish Bolshevik élite ascriptive Jewishness was a social fact mediated by ethnopolitical context, and therefore a dimension of varying significance to their radicalism, even for those for whom Jewishness was not a claimed identity.