The surface structure of the Pd(100)-c(2 x 2)-K phase formed by adsorption of K at room temperature has been determined by quantitative analysis of low-energy electron-diffraction (LEED) intensity-energy measurements. K atoms occupy four-fold hollow sites on a slightly perturbed substrate. The vertical distance between the K layer and the first Pd layer is determined to be 2.13 +/- 0.06 Angstrom, which corresponds to an effective hard-sphere radius of 1.83 Angstrom for the adsorbed K atoms. The second Pd layer is rumpled with a splitting of 0.04 Angstrom between two bilayers. An analysis of LEED intensities measured for the clean Pd(100) surface confirms previous reports of an expansion of the first interlayer spacing. Adsorption of K in the c(2 x 2) structure results in a reduction of this expansion from 5% to 1%.