The cluster-nuclei coassembly strategy, first reported by Wang et al. in 2019, refers to the coassembly of polyoxometalate (POM) clusters and inorganic nuclei affording ultrathin/subnanometer (sub-1 nm) materials such as nanowires, nanobelts, nanosheets, and nanotubes. These materials exhibit unique properties and hold promises for applications in catalysis, energy storage, photothermal conversion, and tumor treatment. This review discusses progress in cluster-nuclei coassembled ultrathin materials and covers their synthesis methods, well-defined structures, and application prospects, aiming to provide a roadmap for the future design and further development of sub-1 nm POM-nuclei coassembly systems.