The water vapor transmission rates (WVTRs) of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (PHBV) and PHBV/nanocomposite-coated papers were measured at various levels of relative humidity and temperature. The coating of PHBV on base paper was performed with two different methods, and the more effective one for lowering WVTR values was utilized for coating the PHBV/nanocomposites on the paper. Nanocomposites of PHBV were prepared with a commercial montmorillonite Cloisite30B (CS30B), and a novel modified clay was obtained via a solution-blending process. To prepare the series of modified clays, which were coined here as HPGC, MPGC, and LPGC, we intercalated beta-butyrolactone in the Cloisite30B clay by a ring-opening polymerization. The morphology of the clay and nanocomposites was revealed by X-ray diffraction. Additionally, the dispersion and phase behavior of the clay in the PHBV matrix was observed using a transmission electron microscope. It was found that the coating method and clay exfoliation were the most important factors affecting water vapor permeability. The water vapor barrier of the coated papers was improved significantly if the surface of the base substrate was prespray-coated with a PHBV suspension prior to the laminate coating of the PHBV film with a hot press. The papers coated with exfoliated PHBV/nanocomposites exhibited even lower WVTR values. Overall, PHBV, PHBV/CS30B, and PHBV/HPGC coating treatments lowered the WVTR values by 46, 56, and 118 times, respectively. The resulting coated paper is promising as a green-based packaging material due to an improved moisture barrier.