Targeted drug delivery offers the possibility to greatly improve treatment outcomes for diseases such as cancer by enhancing specificity, thus minimizing detrimental side effects from off target delivery. Polymer vesicles, or polymersomes, have been shown to be a promising delivery vehicle capable of encapsulating a variety of hydrophilic and hydrophobic drugs in a nanoscale, long-circulating carrier. We have synthesized and thoroughly characterized the end-functionalized and amphiphilic diblock polymer poly(ethylene oxide)-block-poly(γ-methyl-ε-caprolactone). This block copolymer self-assembles in water to form polymersomes containing the vinyl sulfone electrophile at the poly(ethylene oxide) corona terminus. Following polymersome self-assembly, this functional group underwent efficient, site-selective attachment to a thiol-containing targeting peptide by a conjugate addition reaction under mild conditions and did not react with targeting peptides lacking a thiol. The vesicle morphology was unaltered following peptide addition. This work develops a versatile platform for the targeted delivery of therapeutics.