Based on the spatiotemporal modulation of thermal conductivity and volumetric heat capacity, we propose a thermal-wave diode characterized by the rectification of the heat currents carried by thermal waves. By transforming Fourier’s law for the heat flux and the diffusion equation for the temperature into equations with constant coefficients, it is shown that: (i) the rectification effect is generated by the simultaneous wavelike modulation of both thermal properties, such that it disappears in the absence of either of them, and (ii) the rectification factor can be optimized and tuned by means of the speed and phase difference of the variations of the heat capacity and thermal conductivity. High rectification factors, greater than 86%, are obtained for lower frequencies driving the propagation of thermal waves. The proposed thermal-wave diode is thus analogous to its electronic counterpart operating with modulated electrical currents and can open a vista for developing different types of thermal-wave logic components.