<p>The heat generated within integrated circuit architectures needs to be efficiently transferred to the package surface through multilayer interfaces in the out-of-plane direction. At the same time, reverse heat flow needs to be blocked to prevent the failure of thermally sensitive components. Achieving the necessary asymmetric phonon transport in the out-of-plane direction is, however, challenging. Here we report a trilayer van der Waals heterostructure that offers asymmetric thermal transport. This thermal Janus crystal is made of molybdenum disulfide (MoS₂)/molybdenum sulfide selenide (MoSSe)/tungsten diselenide (WSe₂) and has a thickness of less than 5 nm. By precisely adjusting the twist angles at the MoS₂/MoSSe and MoSSe/WSe₂ interfaces, the asymmetric property—defined as the relative change in the interfacial thermal conductance under opposite temperature gradients—can be tuned from 23% to 104%. Molecular dynamics simulations indicate that the contributions of in-plane and out-of-plane phonon modes at the MoS₂/MoSSe and MoSSe/WSe₂ interfaces are different, which leads to asymmetry in the heat transport. In a thermal test with a field-effect transistor and using a 58-mW microwire heater, the surface temperature is reduced by 3.9 K when heat flows from WSe₂ to MoS₂ compared with when the heat flows in the opposite direction through the heterostructure.</p>