In an effort to impart light gas (i.e., H-2 and He) barrier to polymer substrates, thin films of polyethylenimine (PEI), poly(acrylic acid) (PAA), and montmorrilonite (MMT) clay are deposited via layer-by-layer (LbL) assembly. A five quadlayer (122 nm) coating deposited on 51 m polystyrene is shown to lower both hydrogen and helium permeability three orders of magnitude against bare polystyrene, demonstrating better performance than thick-laminated ethylene vinyl-alcohol (EVOH) copolymer film and even metallized polyolefin/polyester film. These excellent barrier properties are attributed to a nanobrick wall structure. This highly flexible coating represents the first demonstration of an LbL deposited film with low hydrogen and helium permeability and is an ideal candidate for several packaging and protection applications.