In general, a three degree-of-freedom (DOF) horizontal motion control has been regarded as adequate in the design of a dynamic positioning system (DPS). However, for certain marine constructions, especially those with a small waterplane area and low metacentric height, which results in relatively low hydrostatic restoring compared to the inertia forces, an unintentional coupling phenomena between the vertical plane and the horizontal plane through the thruster action can be invoked. A novel roll–pitch motion controller with horizontal motion control law is proposed in this paper. The roll and pitch proportional controller is first introduced into the DP control law. The performance of the newly proposed controller was demonstrated by time-domain simulations for a dynamically positioned semi-submersible.