We report a new surface modification approach of upconversion nanoparticles (UCNPs) structured as an inorganic host NaYF4 codoped with Yb3+ and Er3+ based on their encapsulation at a two-stage process of precipitation polymerization of acrolein under alkaline conditions in the presence of UCNPs. The use of tetramethylammonium hydroxide both as an initiator of the acrolein polymerization and agent for the UCNP hydrophilization made it possible to increase the polyacrolein yield up to 90%. This approach enabled facile, lossless embedment of UCNPs into the polymer particles suitable for bioassay. These particles are readily dispersible in aqueous and physiological buffers, exhibiting excellent photoluminescent properties, chemical stability, and allowed control of the particle diameters. The feasibility of as-produced photoluminescent polymer particles mean-sized 260 nm for in vivo optical whole-animal imaging was also demonstrated using a home-built epi-luminescence imaging system.