Plasmonics, especially the localized surface plasmon resonance (LSPR) in non-noble metal bismuth nanoparticles (Bi NPs), and its spectral features and applications have stimulated increasing research interest in recent years. However, the lack of mature methods to prepare Bi NPs with a well-controlled size and/or shape significantly limits the experimental investigations concerning the LSPR optical properties. Herein, we realize the size-tunable synthesis of nearly monodisperse spherical Bi NPs through a thiolate pyrolysis reaction in solution. The instantaneous thermolysis of a layered molecular intermediate, bismuth dodecanethiolate [Bi­(SC12H25)3], results in a classical LaMer mechanism for the nucleation and growth of Bi NPs, allowing for a precise size control from 65 to 205 nm in the average diameter. The diameter tunability enables a systematic study on the size dependence of LSPR optical properties of Bi NPs, and we observe rich ultraviolet–visible–near-infrared spectral responses arising from the LSPR absorption and scattering of Bi NPs as their size varies, which will greatly benefit the light harvesting and manipulation in the solar spectrum. Furthermore, we find that a complete oxidation occurs to Bi NPs under air flow at the temperature when they melt and accordingly generate metastable tetragonal-phase β-Bi2O3 NPs that show an optical band gap of 2.15 eV and interesting temperature-dependent β → α → δ → (γ + β) polymorphic transitions.