Messenger ribonucleic acids (mRNAs) are considered as promising alternatives for transient gene therapy, but to overcome their poor pharmacokinetic properties, smart carriers are required for cellular uptake and stimuli-responsive release. In this work, a synthetic concept toward reductive decationizable cationic block copolymers for mRNA complexation is introduced. By combination of RAFT block copolymerization with postpolymerization modification, cationic block copolymers are generated with disulfide-linked primary amines. They allow effective polyplex formation with negatively charged mRNA and subsequent release under reductive conditions of the cytoplasm. In first in vitro experiments with fibroblasts and macrophages, tailor-made block copolymers mediate cellspecific mRNA transfection, as quantified by polyplex uptake and mRNA-encoding gene expression. Furthermore, RAFT polymerization provides access to heterotelechelic polymers with orthogonally addressable endgroup functionalities utilized to ligate targeting units onto the polyplex-forming block copolymers. The results exemplify the broad versatility of this reductive decationizable mRNA carrier system, especially toward further advanced mRNA delivery applications.