The present invention is a clip for use with a strut that at least partially surrounds a hanger rod for the purpose of providing stiffness to the hanger rod. The clip includes a handle portion, a rod-clamping portion, and a fulcrum spaced away from the handle portion. The fulcrum is for being mounted to the strut. The rod-clamping portion is for forcing the hanger rod into tight engagement with an engagement surface of the strut in response to the handle being actuated to an operational position. The present invention also relates to a hanger rod stiffening system that uses a clip and a strut, whereby a handle of the clip is actuated to force a rod-clamping portion of the clip into tight engagement with an internal surface of the strut.