Messenger RNA (mRNA) provides a promising alternative to plasmid DNA as a genetic material for delivery in non-viral gene therapy strategies. However, it is difficult to introduce mRNA in vivo mainly because of the instability of mRNA under physiological conditions. Here, mRNA-protamine complex encapsulated poly(ε-caprolactone) (PCL) nanoparticles (NPs) are proposed for the intracellular delivery of mRNA molecules. The nanoparticles with a size of about 247 nm in diameter have a core-shell structure with an mRNA-containing inner core surrounded by PCL layers, providing high stability and stealth properties to the nanoparticles. The partial neutralization of the negatively charged mRNA molecules with the cationic protamine allows one to modulate the release kinetics in a pH-dependent manner. At pH 7.4, mimicking the conditions found in the systemic circulation, only 25% of the mRNA is released after 48 hours post incubation, whereas at pH 5.0, recreating the cell endosomal environment, about 60% of the mRNA molecules are released within the same time window post incubation. These NPs show no cytotoxicity to NIH 3T3 fibroblasts, HeLa cells and MG63 osteoblasts up to 8 days of incubation. Given the stability, preferential release behavior, and well-known biocompatibility properties of PCL nanostructures, our non-viral PCL nanoparticles are a promising system that simultaneously resolved the two major problems of mRNA introduction and the instability, opening the door to various new therapeutic strategies using mRNA.