Prestressing (PS) tendons are the main load-carrying components of post-tensioned box-girders bridges. Structural damage (e.g. corrosion and broken wires) as well as loss of prestress in the tendons are critical for the performance of the structure and may lead to failure. This paper presents a structural health monitoring approach based on nonlinear ultrasonic guided waves for monitoring prestress levels in 7-wire PS tendons. The nonlinear ultrasonic behavior of the tendon under changing levels of prestress is monitored by tracking higher-order harmonics at (nco) arising under a fundamental guided-wave excitation at (co). Experimental tests on a large-scale single-tendon PT joint specimen, subjected to multiple load cycles, will be presented to validate the monitoring of PS loads through nonlinear ultrasonic probing. Issues and potential for the use of such techniques to monitor post-tensioned bridges in the field will be discussed.