Sub-1 nm materials (SNMs) feature large specific surface areas and near-100% atomic exposures, thus offering abundant exposed active sites and properties well suited for applications in optics, electricity, catalysis, etc. In particular, polyoxometalate (POM)-based SNMs hold great promise for energy storage/conversion applications because of their nanoscale dimensions and synergetic effect of multiple components. Despite the abundance of reported POM-based SNMs, systematic reviews of their synthesis and energy storage/conversion applications are scarce. This review summarizes progress in the synthesis of POM-based SNMs and their applications in energy storage (e.g., Li-ion, Na-ion, Li–O₂, Zn–O₂, and Li–S batteries) and conversion (e.g., photothermal conversion, solar steam generation, and seawater desalination). Moreover, we discuss the related challenges and future perspectives, providing new insights into the rational design and development of efficient POM-based SNMs suitable for energy storage and conversion.