This paper proposes a novel steerable locomotion controller for six-strut tensegrity robots. Tensegrity robots are lightweight and have many promising features such as robustness, shape-shifting capabilities, and deployability, making them good candidates for exploration and scouting of remote areas. Despite these advantages, tensegrity robots are challenging to control due to their large number of degrees of freedom, nonlinear dynamics, and intrinsic compliance. Recently, many step-wise motion controllers have been employed to simplify the locomotion problem, thanks to the discrete nature of the tensegrity structure. In this paper we present a novel locomotion controller which will steer the direction of motion of a six-strut tensegrity robot when used in conjunction with any preexisting step-wise controller. We validated our controller on the SUPERball v2 robot, showing straight and curved trajectories, and an example of navigation around obstacles. Our method is computationally inexpensive, only requires knowledge about the current base triangle (e.g, via accelerometer data), and can be generalized to any six-strut tensegrity robot which can perform step-wise locomotion.