Paired pectoral fins that are capable of rowing motions are an important actuation mechanism for robotic fish. Existing work in this area typically adopts a rigid connection between the actuator and the pectoral fins, which requires a faster actuation speed in the power stroke than in the recovery stroke to produce a net thrust or moment. In addition to increasing the control complexity, the latter requirement leads to slow robot speeds due to prolonged deceleration during the recovery stroke. In this paper, we propose the design of a novel flexible passive joint that connects the servomotor arm to the pectoral fin, to overcome the aforementioned problem. A dynamic model is developed for the joint and for a robotic fish equipped with such joints. The design and the model are evaluated with extensive experimental results. With symmetric actuation patterns during the power and recovery strokes, the robotic fish with the proposed joints shows clear speed advantage over the case involving rigid joints and asymmetric actuation. Motivated by the need for design optimization, the model is further utilized to investigate the influence of the joint length and stiffness on the robot locomotion performance and efficiency. It is found that, for low fin-beat frequencies, longer or more flexible joints lead to higher speeds, and the trend is reversed at high fin-beat frequencies. On the other hand, while the mechanical efficiency shows a decreasing trend when the frequency increases, it is higher with shorter joints. These findings suggest the utility of the proposed model for multiobjective design of the joint and its operating frequency.