Quasi-spherical VO₂ nanoparticles with uniform size and high crystallinity are ideal functional materials for applications in field-effect transistors, smart window coatings and switches. However, the synthesis of these VO₂ nanoparticles has long been a challenge. This article presents a novel doping strategy for the simultaneous control of the size, morphology and polymorphology of VO₂ nanoparticles. Doping can induce the change in crystal structure and exhibits a significant promoting effect on the formation of doped monoclinic VO₂ (VO₂ (M)). Specifically, by antimony (Sb³⁺) doping, hexagonal-shaped, well crystalline monoclinic VO₂ nanoparticles with tunable sizes (8–30 nm) and controllable polymorphs were synthesized via a one-pot, hydrothermal method. Sb³⁺ dopants, which are larger in radius and lower in valence than V⁴⁺ ions, can introduce extra oxygen vacancies during the nucleation and growth of VO₂ nanoparticles. These positively charged nuclei may suppress the adsorption of VO²⁺ aqua ions, and therefore inhibit the growth of the VO₂ (M) nanoparticles. Comparably, Sb⁵⁺ dopants that possess higher valence counts than V⁴⁺ ions can induce the growth of VO₂ (M) particles to 200–300 nm width and above 500 nm length. The Sb³⁺-doped VO₂ (M) nanoparticles exhibit excellent properties in metal–semiconductor transformation at transition temperatures ranging from 55–68 °C. Films obtained by casting these nanoparticles show excellent optical properties (both visible transmittance and infrared regulation), compared with those prepared from gas phases, such as sputtering. This synthetic strategy that involves the doping of an element with a different valence count than the matrix cation may be useful for controlling the solution growth of some technologically significant nanomaterials. In addition, the formation mechanism of solid and crystalline transformation was also studied by designing a specific reaction autoclave.