A system is described for reinforcing a thermoplastic workpiece including a subject surface and an underlying workpiece body volume. At least one substantially linear Z-pin having proximal and distal pin ends longitudinally separated by a pin body is provided. The proximal pin end is in direct contact with the subject surface. An ultrasonic energy source applies ultrasonic energy to the Z-pin to ultrasonically heat the Z-pin and thus locally melt the workpiece material of the subject surface and/or the workpiece body to create a melted workpiece material. The proximal pin end and at least a portion of the pin body of the Z-pin are penetrated into the melted workpiece material to create an inserted Z-pin length. The inserted Z-pin length is maintained in the workpiece body volume by solidified melted workpiece material around the inserted Z-pin length to reinforce the workpiece. A method of reinforcing a workpiece is also provided.