Using surface extended x-ray-absorption fine-structure spectroscopy we establish the K-Si bond length to be 3.14\ifmmode\pm\else\textpm\fi{}0.10 \AA{} at the K/Si(100) 2\ifmmode\times\else\texttimes\fi{}1 interface for (1/2-monolayer K coverage. This value corresponds to the sum of the Si and K covalent radii and suggests weak interaction of K with the substrate relative to the interaction along the K chains on the surface. This finding, while consistent with earlier studies of this system, disagrees with recent total-energy pseudopotential calculations which determine a K-Si distance of 2.59 A\r{}.