Over the past decade, a lot of attention has been paid to plasma actuators because they are useful tools for flow control. Previous successes with plasma actuators include drag and noise reductions from a circular cylinder, one of representative bluff bodies, mainly through separation delay. However, to the best of authors’ knowledge, no attempt has been made to examine its capability to control square cylinder wake with fixed separation points. The purpose of this study is to numerically investigate whether or not the square cylinder wake can be controlled by means of plasma actuators. In particular, a linear plasma synthetic jet actuator is adopted and attached to the rear side of the cylinder. In this study, we use an immersed boundary method combined with an empirical plasma model for plasma-based flow control. The present method is second order accurate in time and space. Results show that the wake behind the square cylinder can be controlled effectively when the plasma-induced force is strong enough. With the plasma actuator on, the mean drag is reduced and the Karman vortex shedding is alleviated because the induced jet increases the base pressure and prevents the separating shear layers from interacting with each other.