Bioimaging plays a vital role in understanding complex biological processes, from tracking cellular activity to studying entire organisms. To improve the sensitivity and specificity of imaging techniques, researchers are increasingly turning to nanomaterials as contrast agents. Among them, upconversion nanoparticles (UCNPs) stand out for their ability to convert near-infrared (NIR) light into visible emission. This property enables deep tissue penetration, reduced autofluorescence, and low phototoxicity. We report the synthesis of LaF codoped with 2 mol% Er and 20 mol% Yb using a hydrothermal method, followed by surface modification with human serum albumin (HSA). Structural analysis confirms the formation of rhombohedral LaF with uniform dopant incorporation, and electron microscopy reveals a near-spherical morphology with average particle sizes increasing from ~25 nm (bare) to ~60 nm (HSA-coated). Under 976-nm excitation, the nanophosphors exhibit bright green and red UC emissions via a two-photon mechanism. Cytotoxicity studies demonstrate high cell viability at concentrations up to 0.4 mg/mL in HeLa cells, while fluorescence uptake assays indicate time-dependent internalization, peaking at 6-h post-treatment. These findings highlight the potential of HSA-coated LaF:Er,Yb nanophosphors as efficient, biocompatible contrast agents for NIR-excited luminescent bioimaging.