A novel poly(ester) urethane system filled with poly(ester) urethane grafted with hollow glass microspheres was fabricated via a melt mixing method for use as an underwater encapsulant. The poly(ester) urethane/poly(ester) urethane grafted with hollow glass microsphere (HGM) composites (PU/PU-g-HGM composites) were prepared using various PU-g-HGM contents. Studies of these composites showed that the swelling ratio of PU/PU-g-HGM composites in seawater and paraffin oil decreased with increasing HGM content of grafted poly(ester) urethane, while the tensile strength of PU/PU-g-HGM composites could be incrementally increased by rising PU-g-HGM content. This finding is due to the enhanced interfacial adhesion between poly(ester) urethane and poly(ester) urethane-grafted HGM due to good compatibility between these components. The tensile strength of the PU/PU-g-HGM composites in seawater (or paraffin oil) also exhibited higher levels than those of poly(ester) urethane and PU/ pristine HGMs composites in seawater (or paraffin oil) as the immersion time increased. Therefore, the PU/PU-g-HGM composites were expected to have potential applications as underwater encapsulants.