The hydrodynamic coefficients of a semisubmersible undergoing slow-drift oscillation were determined through a model test. The semisubmersible model was moored in head seas, fore and aft, with linear springs which were pretensioned and never became slack during any test run. At the beginning of each test run, the vessel was held at an initial displacement from its equilibrium position and then released, and the resulting line loads were recorded. The semisubmersible was tested in still water and in regular waves. The amplitude of the waves at a given period was varied. The added mass and damping of the semisubmersible were determined from the decayed oscillation of the loads. The semisubmersible experienced both linear and nonlinear damping. The hydrodynamic coefficients obtained from the semisubmersible as functions of wave height and period are compared with those found previously on a tanker.