<p>Heat flow in semiconductors typically occurs through the diffusive transport of lattice vibrations. Non-diffusive hydrodynamic effects associated with viscous heat flow and thermoelastic effects—in which heat changes the interatomic spacing in the lattice—can affect heat conduction. However, the interplay between hydrodynamic and thermoelastic effects on heat transport has so far been overlooked. Furthermore, unconventional thermoelastic effects due to inhomogeneous strain fields at the nanoscale have so far not been observed experimentally. Here we show that the combination of hydrodynamic and thermoelastic effects leads to a highly non-diffusive hydro-thermoelastic heat transport regime with a controllable reduction in the effective thermal diffusivity for two-dimensional semiconductors. We observe this in MoSe₂ and MoS₂ through real-space heat tracking with nanometre spatial accuracy using spatiotemporal pump–probe thermometry. Our experiments are conducted at room temperature and demonstrate control through the thickness of the material and by choosing continuous or pulsed heating. Our model of hydro-thermoelastic heat transport, based on atomistic input parameters, reproduces the experimental observations and identifies the occurrence of a counterintuitive thermoelastic heat flux contribution from cold to hot regions.</p>