Polymersomes, polymeric vesicles that self-assemble in aqueous solutions from block copolymers, have been avidly investigated in recent years as potential drug delivery agents. Past work has highlighted peptide-functionalized polymersomes as a highly promising targeted delivery system. However, few reports have investigated the ability of polymersomes to operate as gene delivery agents. In this study, we report on the encapsulation and delivery of siRNA inside of peptide-functionalized polymersomes composed of poly(1,2-butadiene)-b-poly(ethylene oxide). In particular, PR_b peptide-functionalized polymer vesicles are shown to be a promising system for siRNA delivery. PR_b is a fibronectin mimetic peptide targeting specifically the alpha(5)beta(1) integrin. The Orai3 gene was targeted for siRNA knockdown, and PR_b-functionalized polymer vesicles encapsulating siRNA were found to specifically decrease cell viability of T47D breast cancer cells to a certain extent, while preserving viability of noncancerous MCF10A breast cells. siRNA delivery by PR_b-functionalized polymer vesicles was compared to that of a current commercial siRNA transfection agent, and produced less dramatic decreases in cancer cell viability, but compared favorably in regards to the relative toxicity of the delivery systems. Finally, delivery and vesicle release of a fluorescent encapsulate by PR_b-functionalized polymer vesicles was visualized by confocal microscopy, and colocalization with cellular endosomes and lysosomes was assessed by organelle staining. Polymersomes were observed to primarily release their encapsulate in the early endosomal intracellular compartments, and data may suggest some escape to the cytosol. These results represent a promising first generation model system for targeted delivery of siRNA.