In the quest for low-temperature polyolefins upcycling, overcoming the decomposition temperature limitations caused by chemically inert carbon-hydrogen (C(sp³)–H) bonds remains a challenge. Here we report a tandem endothermic-exothermic magnetocatalytic (EEM) strategy for upcycling of polypropylene (PP) and polyethylene (PE) below their decomposition temperature (typically 300 °C). A series of commodity polyolefins (PP, PE, and their mixtures (PP+PE)) can be deconstructed magnetocatalytically at 290 °C, for which a high H₂ selectivity up to 86% can be achieved. Magneto-thermal coupling simulations revealed that the interfacial temperature gap was the key to upcycling of polyolefins at low temperature. Overall, EEM method potentially provides a sustainable approach for end-of-life plastics upcycling.