We demonstrate a route to synthesize ultra-high-temperature ceramic coatings of ZrB2 at temperatures below 1300 K using Zr/B reactive multilayers. Highly textured crystalline ZrB2 is formed at modest temperatures because of the absence of any oxide at the interface between Zr and B and the very short diffusion distance that is inherent to the multilayer geometry. The kinetics of the ZrB2 formation reaction is analyzed using high-temperature scanning nanocalorimetry, and the microstructural evolution of the multilayer is revealed using transmission electron microscopy. We show that the Zr/B reaction proceeds in two stages: (1) interdiffusion between the nanocrystalline Zr and the amorphous B layers, forming an amorphous Zr/B alloy, and (2) crystallization of the amorphous alloy to form ZrB2. Scanning nanocalorimetry measurements performed at heating rates ranging from 3100 to 10000 K/s allow determination of the kinetic parameters of the multilayer reaction, yielding activation energies of 0.47 and 2.4 eV for Zr/B interdiffusion and ZrB2 crystallization, respectively.