Two-dimensional semiconductors could be used as a channel material in low-power transistors, but the deposition of high-quality, ultrathin high-κ dielectrics on such materials has proved challenging. In particular, atomic layer deposition typically leads to non-uniform nucleation and island formation, creating a porous dielectric layer that suffers from current leakage, particularly when the equivalent oxide thickness is small. Here, we report the atomic layer deposition of high-κ gate dielectrics on two-dimensional semiconductors using a monolayer molecular crystal as a seeding layer. The approach can be used to grow dielectrics with an equivalent oxide thickness of 1 nm on graphene, molybdenum disulfide (MoS₂) and tungsten diselenide (WSe₂). Compared with dielectrics created using established methods, our dielectrics exhibit a reduced roughness, density of interface states and leakage current, as well as an improved breakdown field. With the technique, we fabricate graphene radio-frequency transistors that operate at 60 GHz, and MoS₂ and WSe₂ complementary metal–oxide–semiconductor transistors with a supply voltage of 0.8 V and subthreshold swing down to 60 mV dec⁻¹. We also create MoS₂ transistors with a channel length of 20 nm, which exhibit an on/off ratio of over 10⁷.