The effect of top tension risers on the spar marine structure responses is studied by using time domain as well as frequency domain approaches. A simplified model of the combined dynamic system consisting of a spar, a mooring line system, and top tension risers is established. The top tension of a riser is provided by a buoyancy-can that is laterally supported by a preloaded compliant guide. The overall system is nonlinear due to the mooring line restoring force, the friction between the buoyancy-can and the compliant guides, and the friction between the risers and guides at the spar keel. The friction between the spar and riser system is represented via a continuous Coulomb model. The simplified model is numerically integrated using input time histories of force and moment that are compatible with a random representation (Jonswap spectrum) of a typical extreme environmental loading in the Gulf of Mexico. For validation, the dynamic responses of the simplified system are compared with the related responses of the combined system. The effect of the top tensioned risers on the spar responses is studied by using nonlinear time domain analysis. The responses of the simplified model are also determined by using a frequency domain approach based on the statistical linearization technique. Several parametric studies are conducted to investigate the sensitivity of the spar responses to different design parameters.