The article takes its starting point from contemporary descriptions of Augustan rule, which according to Tacitus argued that Augustus had re-established the Roman res publica under the name princeps. While the presence of republican and monarchical elements in the principate was broadly discussed in late 19th and early 20thcentury, it is nearly ignored in current research: There the Roman emperors, in spite of all their peculiarities, are portrayed as regular monarchs similar to those of other historical epochs and cultures. In contrast to that view, it is argued here that the Roman res publica was no ‘republic’ in terms of modern constitutionalism, but rather a form of political organisation which at the same time reproduced the rank system of the aristocratic society. Therefore, its social function made it irreplaceable also under imperial rule. This led to a paradoxical coexistence of traditional institutions – republican offices and senate, which as such delegitimised any forms of monarchical rule – and new structures of patrimonial rule based on military power. Both were inevitable, yet incompatible. The results of this study explain underexposed but crucial elements of the principate and challenge well-accepted views of Roman imperial ‘monarchy’.