<p>Non-van der Waals (vdW) two-dimensional (2D) metal oxides provide an opportunity to enrich the current 2D material library and accelerate the adoption of 2D materials into future electronic devices. However, the exploration of 2D non-vdW oxides remains limited, constrained by synthetic challenges and insufficient fundamental understanding. Here, we report a 2D wide-bandgap non-vdW oxide family—A₂Mo₃O₈ (A = Zn, Mn, Co, etc.), known as kamiokite-type oxides. A dual-controlled chemical vapor deposition strategy enables the in-plane growth of ultrathin A₂Mo₃O₈ single crystals with thicknesses below 3 nm and domain sizes exceeding 100 μm, overcoming the intrinsic constraints of lattice isotropy while providing facile and precise control over product stoichiometry. Notably, 2D Zn₂Mo₃O₈ exhibits a bandgap of 4.0 eV and a high dielectric constant (<i>κ</i> ~ 25). Benefiting from the good dielectric properties and high-quality interface of Zn₂Mo₃O₈, MoS₂ transistors with a Zn₂Mo₃O₈ top-gate dielectric exhibit near-Boltzmann-limit subthreshold swing (~63.8 mV dec⁻¹), an on/off ratio exceeding 10⁷, and an ultralow hysteresis width of 1.6 mV MV⁻¹ cm under ambient conditions. The A₂Mo₃O₈ family offers a versatile materials platform for the development of high‑<i>κ</i> dielectrics and a broad range of functional components for next-generation high-performance 2D electronics.</p>