Multiple myeloma is a common plasma-cell-derived hematologic neoplasm. While the delivery of growth-inhibiting miRNA to multiple myeloma cells would be a promising strategy to evaluate treatment options, most multiple myeloma cells are transfection-resistant with established methods. Nonviral nano-particulate transfection systems are particularly promising in this context, but so far struggle with transfection and knockdown efficiency. Here, we present poly(glycidol)-based nanogels with covalently bound cell-penetrating peptide TAT (transactivator of transcription from HIV). TAT facilitated a varying internalization efficiency of the nanogels depending on the cell line. The positively charged peptide also served as complexation agent for miRNA and enabled covalent binding of the TAT/miR-34a complex in the nanogels. These TAT/miRNA-loaded nanogels delivered and released miR-34a with high efficiency into OPM-2 multiple myeloma cells that are known as transfection-resistant. Delivery resulted in efficient downregulation of known target genes such as Notch1, Hey1, Hes6, and Hes1. Thus, these nanogel constructs offer a new tool to enhance gene delivery into multiple myeloma cells with immediate value in cancer research.