<p>A promising strategy for further miniaturizing metal–oxide–semiconductor field-effect transistors is the use of ultrathin two-dimensional channel materials. However, achieving robust dielectric integration with a sub-1-nm capacitance equivalent thickness (CET) remains challenging. Here we present a wafer-scale monolayer MoO₃, transformed from MoS₂, which can be seamlessly integrated with atomically thin semiconductors. Its atomically flat surface and the strong electronegativity of Mo⁶⁺ further enable the uniform deposition of high-<i>κ</i> dielectrics. Utilizing the 0.96-nm-CET MoO₃/HfO₂ as the dielectric, the top-gated p-type (n-type) two-dimensional transistors show a high ON/OFF ratio of 6.5 × 10⁶ (3.2 × 10⁸) and a steep subthreshold swing of 60.8 (63.1) mV dec⁻¹. Statistical analysis of a 1,024-device array achieves a high yield of 92.2%. Furthermore, when monolayer MoO₃ is used as the top-gated dielectric with an ultimately scaled CET of 0.64 nm, the gate leakage current meets the low-power limit standard (1.5 × 10⁻²A cm⁻²) over the entire bias range. Our study provides a scalable approach for the integration of ultralow-CET dielectrics on two-dimensional materials, marking a critical step towards their future industrial deployment.</p>