<p>Luminescence quenching in aqueous environments poses a challenge for practical applications. Lanthanide-doped up-conversion nanoparticles (UCNPs), representative of near-infrared (NIR)-emitting phosphors, typically utilize Yb³⁺ ions as sensitizers, requiring 980 nm light. This wavelength coincides with the transitions of water molecules, interfering with population dynamics, and continuous irradiation causes unintended heating. Although Nd³⁺ ions, which absorb at 800 nm, serve as alternative sensitizers, their practical use is limited by low quantum yield (Q.Y.). In this study, we developed water-insensitive down-shifting nanoparticles (WINPs) functioning within the NIR-I range (700–900 nm) to avoid water interference. Characterization through single-particle-level spectroscopy demonstrated water-insensitive properties, with identical powers density and lifetime profiles under both dry and water conditions. The WINPs achieved a high Q.Y. of 22.1 ± 0.9%, allowing operation at a detection limit power 15-fold lower than UCNPs, effectively eliminating background noise and enhancing overall performance. To assess diagnostic potential, we validated WINP-based lateral flow immunoassay (LFA) for detecting avian influenza viruses (AIVs) in 65 opaque clinical samples, achieving 100% sensitivity and an area under the curve (AUC) of 1.000 at only 100 mW cm⁻². These findings highlight the potential of WINPs as water-insensitive NIR phosphors that can operate at low power, even in water-rich environments.</p>