One of the major engineering challenges for the implementation of block copolymer vesicles, or polymersomes, as therapeutic drug carriers is obtaining high encapsulation efficiencies for biomolecules. Here we present a novel method for encapsulation of proteins with high encapsulation efficiency within polymersomes formed from block copolymers of poly(ethylene glycol)-bl-poly(propylene sulfide). By formulation of the neat block copolymer with a low molecular weight poly(ethylene glycol), direct hydration of the formulated mixture yielded polymersomes. We were able to achieve encapsulation efficiencies for ovalbumin at 37%, bovine serum albumin at 19%, and bovine gamma-globulin at 15% when the proteins were included in the hydration solution. The formulation process and the dispersion of polymersomes from the preparation in phosphate-buffered saline were characterized using confocal microscopy, cryogenic transmission electron microscopy, and fluorimetry. We were also successful in the encapsulation of proteinase K, a proteolytic enzyme, and demonstrated by SDS-PAGE that the enzyme was contained inside polymersomes when dispersed in a solution of ovalbumin.