Nanocellulose/montmorillonite (MTM) composite films were prepared from 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibrils (TOCNs) with an aspect ratio of >200 dispersed in water with MTM nanoplatelets. The composite films were transparent and flexible and showed ultrahigh mechanical and oxygen barrier properties through the nanolayered structures, which were formed by compositing the anionic MTM nanoplatelet filler in anionic and highly crystalline TOCN matrix. A composite film with 5% MTM content had Young's modulus 18 GPa, tensile strength 509 MPa, work of fracture of 25.6 MJ m(-3), and oxygen permeability 0.006 mL μm m(-2) day(-1) kPa(-1) at 0% relative humidity, respectively, despite having a low density of 1.99 g cm(-3). As the MTM content in the TOCN/MTM composites was increased to 50%, light transmittance, tensile strength, and elongation at break decreased, while Young's modulus was almost unchanged and oxygen barrier property was further improved to 0.0008 mL μm m(-2) day(-1) kPa(-1).