Although microRNAs (miRs) are short endogenous noncoding RNAs playing a central role in cancer initiation and progression, their therapeutic potential in overcoming multidrug resistance (MDR) remains unclear. In the present study, we developed self-cross-linked biodegradable poly(ethylene glycol)-b-poly(L-lysine)-b-poly(L-cysteine) (LCss) polypeptide nanoparticles to codeliver DOX and miR-129-5p, which aimed to overcome MDR in cancer cells. The results showed that LCss nanoparticles effectively coencapsulated DOX and miR with great stability, but quickly disassembled and released their payload in a bioreducible environment. The codelivery of miR-129-5p and DOX with LCss (DLCss/miR) significantly increased miR-129-5p expression over 100-fold in MCF-7/ADR cells, which effectively overcame MDR by directly inhibiting P-glycoprotein (P-gp), thereby increasing intracellular DOX accumulation and cytotoxicity in MCF-7/ADR cells. Furthermore, miR-129-5p also partially diminished cyclin-dependent kinase 6 (CDK6), and synergized with DOX to simultaneously decrease S phase and induce G(2) phase cell cycle arrest, thereby further enhancing the chemosensitivity of MCF-7/ADR cells. Hence, redox-responsive LCss nanoparticles are potent nanocarrier for combinational drug-miR therapy, which could be a promising strategy to overcome MDR in cancer cells.