A method for producing a polymer nanocomposite, includes: after expanding an interlayer space of a layered inorganic compound using an organic onium salt, immobilizing a radical polymerization initiator having a group represented by formula (1) on an interlayer surface of the layered inorganic compound via a covalent bond; and carrying out surface-initiated radical polymerization in a monomer from the immobilized radical polymerization initiator. Preferably, in the radical polymerization initiator having the group represented by the general formula (1), at least one of X1 to X3 in the formula is a chlorine atom, or R1 in the formula is a C1-25 alkylene group or a C2-25 alkenylene group optionally interrupted by an oxygen atom or an ester group and optionally substituted by a phenyl group: R1 represents a C1-25 alkylene group, a C2-25 alkenylene group, a C5-8 cycloalkylene group, or a C6-12 arylene group.