We perform first-principles calculations to determine the atomic and electronic properties of the Sb-Ge(lll)(2X1) surface. The Sb atoms are located on top positions forming zigzag chains. In agreement with x-ray diffraction experiments, we found that the substrate Ge lattice exhibits only small distortions, and the Sb chain is slightly tilted. Structural parameters are in excellent agreement with x-ray diffraction, x-ray standing waves, and surface extended x-ray absorption fine structure experiments.