Abstract: The robotic ribbon fin is a propulsive mechanism inspired by the long fin of the agile, South American black ghost knifefish. The natural ribbon fin can produce rapid accelerations in any of multiple directions, enabling the fish to quickly reach points within the range of its weakly electric sensing system. We discuss the design and control of the robotic ribbon fin and introduce a computational model of the thrust which will be useful for closed-loop control and gait optimization algorithms. The robotic ribbon fin has potential application in future underwater vehicles that require a high degree of maneuverability, rapid accelerations and effective station-keeping.