While fully biodegradable poly(butylene adipate-coterephthalate) (PBAT) is considered to be one of the most promising materials, the application of PBAT materials is still hindered due to its intrinsic low mechanical strength and poor foamability. Herein, a strategy is addressed to construct PBATbased supramolecular bionanocomposites of high strength and excellent foamability with homogeneous dispersion of UPy aggregates via quadruple hydrogen bonding. The biocomposite was prepared by direct melt-blending of isocyanate-terminal ureidopyrimidinone (UPy-NCO) and PBAT. It was found that the terminal hydroxyl group of PBAT quickly reacted with the isocyanate group of UPy-NCO during the mixing and in-situ formed a dimerized supramolecular polymer (PBAT-UPy-UPy-PBAT) owing to quadruple hydrogen bonding. The PBAT-UPy-UPy-PBAT could be further folded via longitudinal hydrogenbonding interactions between the ureido groups of UPy units, forming ordered needle-like UPy nanoassemblies in the PBAT matrix. Such particular nanoassemblies could function as physical cross-linking points for PBAT-based composites, resulting in simultaneous improved mechanical strength and foamability of PBAT. This work paves a feasible avenue for manufacturing high-performance PBAT biocomposites and corresponding foams, which is challenging in existing industrial technologies.