The flow properties of dilute solutions of linear, star, hyperbranched and dendrimeric polymers have been the subject of numerous studies. However, no systematic analysis has been carried out for the case of single-chain nanoparticles (SCNPs) of different nature, which are unimolecular soft nano-objects consisting of individual polymer chains collapsed to a certain degree by means of intramolecular bonding. On the basis of the fractal nature of SCNPs and experimental data of the hydrodynamic radius, a simple predictive power-law between the intrinsic viscosity and molecular weight is proposed. Furthermore, a comparison is made between the intrinsic viscosities of SCNPs and of low-functionality stars, hyperbranched and dendrimeric polymers of the same chemical nature and molecular weight. As a consequence of their complex nanoscopic architecture, the intrinsic viscosities of SCNPs are systematically smaller than those of linear chains and low-functionality stars. When compared with hyperbranched and dendrimeric polymers, a complex behaviour is found, this being highly dependent on the molecular weight and amount of X-linkers of SCNPs.