To treat cancer cells overexpressing P-glycoprotein (P-gp), we propose a new concept using a nanodrug. The nanodrug was prepared from polyethyleneimine (PEI)/all-trans retinoic acid (ATRA) conjugates (PRA) and covered with hyaluronic acid (HA) to control the cytotoxicity of PRA (yielding PRA-H). The size distribution of PRA-H was narrow, with an average particle size of approximately 143 nm. Its superior stability in phosphate-buffered saline (PBS) was verified by monitoring changes in particle size and zeta potential for 24 h, which were negligible. In contrast, PEI-H (not conjugated with ATRA) exhibited a significant change in particle size and zeta potential. Although PRA was highly cytotoxic against HCT-8 and SNU-484 cancer cells, both of which overexpress P-gp, the cytotoxicity was significantly reduced by shielding with HA. The cytotoxicity of PRA-H was recovered by treatment with hyaluronidase (HAase), which degrades HA and is present in tumors at high concentrations. These results were confirmed by optical microscopy, fluorescence-activated cell sorting (FACs) analysis, and confocal microscopy. The cytotoxic mechanism of PRA was revealed as a type of necrotic lysis by FACs analysis with propidium iodide (PI) staining. Furthermore, PRA increased HCT-8 cell (colon cancer) permeability to doxorubicin (DOX). Therefore, we concluded that PRA-H is a promising new candidate for the treatment of cells with multidrug resistance (MDR) induced by overexpression of P-gp and cancer stem cells.