The X-ray Multi Mirror (XMM) satellite mission is one of the major European Space Agency (ESA) space missions scheduled for launch by the end of this decade. Its goal is to observe the x-ray-emitting sky in the 100 eV to 15 keV photon energy range with high spectral and spatial resolution. XMM's scientific instruments consist mainly of three independent Wolter telescopes each having an imaging charge-coupled-device (CCD) detector in the focal plane. Two of the three telescopes are equipped with CCDs based on the conventional MOS-CCD technology. The third is equipped with pn-CCDs, a new developement of the semiconductor laboratory of the Max-Planck-Institut für extraterrestrische Physik. The pn-CCD has several features such as full detector depletion, parallel readout and good radiation hardness due to the replacement of the MOS structures with pn-diode electrodes. The design of the pn-CCD detector for XMM is shown and measurements with pn-CCD prototype units are presented.