An effective gas barrier nanocomposite coating based on polyvinyl alcohol (PVA) and montmorillonite (MMT) was applied onto two biodegradable polymer substrates, cellulose and poly(propylene carbonate-co-phthalate) (PPC-P), via a dip-coating and flow-induced self-assembly method. The microstructure, transparency, wettability, and gas barrier performance of the coated substrates were systematically investigated. Thanks to the high level of dispersion and orientation of the MMT nanosheets, the PVA/MMT nanocoating with a nacre-like microstructure endowed the coated substrates with significantly improved gas barrier properties. The OTR and WVTR of the coated cellulose/PPC-P films decreased to 0.35/<0.02 cm3/(m2·day) and 13.06/0.23 g/(m2·day), respectively. Moreover, the coated substrates exhibited excellent optical transmittance. Therefore, the PVA/MMT coated cellulose and PPC-P films with excellent gas barrier performance and high transparency show competitive potential for green packaging applications.