The thermal spectrum of the zodiacal light cannot be observed from the ground. We collected 27 mid-infrared (6-12mu m) spectra obtained by the spectrophotometer ISOPHOT-S on-board the Infrared Space Observatory (ISO). The observed positions are distributed over the sky region accessible by ISO. The spectrum of the zodiacal light is remarkably well fitted by a Planck curve of about 270 K over the observed wavelength range. No obvious spectral features are detected. The measurements place a strong constrain on models of interplanetary dust concerning constituents and size distributions. The observed slight dependence of the temperature of the zodiacal light on ecliptic position can be modelled in the context of the three-dimensional density and temperature distribution of the interplanetary dust cloud.