The incorporation of multiple immiscible metals in high-entropy oxides can create the unconventional coordination environment of catalytic active sites, while the high formation temperature of high-entropy oxides results in bulk materials with low specific surface areas. Here we develop the high-entropy LaMnO₃-type perovskite-polyoxometalate subnanowire heterostructures with periodically aligned high-entropy LaMnO₃ oxides and polyoxometalate under a significantly reduced temperature of 100 ^oC, which is much lower than the temperature required by state-of-the-art calcination methods for synthesizing high-entropy oxides. The high-entropy LaMnO₃-polyoxometalate subnanowires exhibit excellent catalytic activity for the photoelectrochemical coupling of methane into acetic acid under mild conditions (1 bar, 25 ^oC), with a high productivity (up to 4.45 mmol g^‒1_cat h^‒1) and selectivity ( > 99%). Due to the electron delocalization at the subnanometer scale, the contiguous active sites of high-entropy LaMnO₃ and polyoxometalate in the heterostructure can efficiently activate C − H bonds and stabilize the resulted *COOH intermediates, which benefits the in situ coupling of *CH₃ and *COOH into acetic acid.