In order to strengthen the interface of a composite scarf joint, this study investigated the benefits of using locally applied carbon nanotubes to reinforce a carbon fiber composite scarf joint. The effect of carbon nanotubes on enhancing the fracture toughness and interface strength was investigated by performing Mode I and Mode II fracture tests with and without carbon nanotubes applied locally at the joint interface. Furthermore, the effects of seawater absorption and different carbon nanotube concentration values on Mode II fracture were investigated. Finally, a partial application of carbon nanotubes only near the crack tip area was considered. During the study, the image correlation technique was used to examine the fracture mechanisms altered by the introduction of carbon nanotubes. The experimental study showed that an optimal amount of carbon nanotubes could increase the fracture toughness of the composite joint interface significantly, especially for Mode II, including a physical change in the fracture mechanism.