We combine geodetic and seismic data sets to constrain the coseismic slip distribution of the Mw6.6 San Simeon earthquake which occurred in the central California Coast Ranges on December 22, 2003. We use continuous and survey-mode GPS observations along with seismic waveform data from the California Integrated Seismic Network (CISN). We invert both data sets for the distribution of finite fault slip in a layered-Earth model. The inversion results indicate that the rupture extends to the southeast of the epicenter for approximately 25--30 km and occurred in a relatively shallow depth range between 1 to 10 km. The average and peak slip are 0.61 m and 2.37 m, respectively. The scalar seismic moment is 7.85e18 Nm, and the static stress drop 1.7 MPa. This thrust earthquake is characterized by its shallow, along-strike extended slip.