AbstractFluorescent nanoparticles are widely utilized in a large range of nanoscale imaging and sensing applications. While ultra‐small nanoparticles (size ≤10 nm) are highly desirable, at this size range, their photostability can be compromised due to effects such as intensity fluctuation and spectral diffusion caused by interaction with surface states. In this article, a facile, bottom‐up technique for the fabrication of sub‐10‐nm hexagonal boron nitride (hBN) nanoparticles hosting photostable bright emitters via a catalyst‐free hydrothermal reaction between boric acid and melamine is demonstrated. A simple stabilization protocol that significantly reduces intensity fluctuation by ≈85% and narrows the emission linewidth by ≈14% by employing a common sol–gel silica coating process is also implemented. This study advances a promising strategy for the scalable, bottom‐up synthesis of high‐quality quantum emitters in hBN nanoparticles.