Boxfishes (Ostraciidae; Tetraodontiformes) have a rigid carapace which restricts body undulation. Swimming movements can only be generated by the fins which protrude from the carapace. Nevertheless, these fishes are highly manoeuvrable and manage to swim with remarkably dynamic stability. However, the rigid carapace of boxfishes shows an inherently unstable response in yaw caused by course-disturbing flows. Hence, any net stabilising effect should come from the fishes’ fins. Here, we aim to determine the effect of the surface area and orientation of the caudal fin on the yaw torque exerted on the square cross-sectional shaped yellow boxfish (Ostracion cubicus). Yaw torques were quantified in a flow tank using a 3D printed physical yellow boxfish model with an attachable closed or open caudal fin. The model was positioned at different body and tail angles and exposed to different water flow speeds. We show that the caudal fin is crucial for yaw control. These flow tank results were confirmed by computational fluid dynamics simulations. The caudal fin acts as both a course-stabiliser and rudder for the naturally unstable rigid carapace with regard to yaw. By using physical models and computer simulations, we quantitatively show that actively changing the shape and orientation of the caudal fin plays an important role in controlling yaw torque in yellow boxfish. Further study is needed to unravel how all components of the boxfishes’ locomotor apparatus function together, from a dynamic perspective, during lateral gust flows and turning.