Fluorescent nanodiamonds (FNDs) containing quantum defects enable the optical measurement of electromagnetic fields and temperature and are among the most developed nanoscale quantum sensors today. Yet, for many applications in biomedicine and beyond, FNDs must be integrated into biocompatible substrates that preserve FND sensitivity and bring FNDs within nanoscale distance of their sensing target. At the same time, the high excitation light intensity required for most quantum sensing protocols remains a major challenge for applications in biomedicine. Here, it is shown that FNDs embedded in polyvinyl‐alcohol (PVA) fibers are a powerful 3D platform for nanoscale quantum sensing via light guiding. First, it is demonstrated that biocompatible PVA fibers can guide light to excite FNDs >10 µm from the excitation beam. Using this light‐guiding‐enabled excitation, optically detected magnetic resonance (ODMR) and T₁ spin relaxometry measurements are performed using the nitrogen‐vacancy (NV) center in FNDs. Through ODMR thermometry, it is shown that light‐guiding‐enabled excitation mitigates light‐induced heating. Finally, the quantum sensing capability of the platform is established by detecting paramagnetic gadolinium in a dry and aqueous environment using T₁ relaxometry of the NV center in FNDs. These results pave the way for light‐guiding‐enabled optical quantum sensing in biomedicine using nanodiamond‐doped biosubstrates.