Porous silicon nanoparticles (PSiNPs) are non-toxic photoluminescence imaging agents whose potential for biophotonic applications has been widely reported in recent years. However, this material suffers from intrinsic limitations, due to its chemical instability and rapid photoluminescence extinction on exposure to the physiological environment. In this letter, a mild functionalization procedure for PSiNP stabilization, based on undecylenic acid conjugation via hydrosilylation, is proven to be effective under simulated biological conditions [phosphate-buffered saline (PBS) 0.1 M, pH = 7.4]. Once stored in isopropanol, bare and hydrosilylated PSiNPs (h·PSiNPs) show similar photoemissive properties: photoluminescence quantum yield (>10%), steady-state spectra, and emission lifetime. Interestingly, the behaviour of these nanoparticles is completely different in the physiological medium: the morphology and the photoluminescence emission of bare PSiNPs completely degrade within 2 h, whereas a full preservation of the same features up to 6 h is demonstrated for h·PSiNPs.