A trailer tire monitoring system and method is employed with a trailer having tires and being towed by a vehicle. The trailer tire monitoring system has tire sensor assemblies, each mounted in a respective one of the trailer tires, with each tire sensor assembly including a sensor apparatus that monitors at least one tire operating parameter and a wireless transmitter apparatus capable of wirelessly transmitting data relating to the at least one tire operating parameter. The monitoring system also has a relay unit preferably mounted on the trailer and including a relay receiver adapted for receiving the wirelessly transmitted data from the wireless transmitters of the tire sensor assemblies, and a wireless relay transmitter that wirelessly transmits the data. An in-vehicle receiver receives the wirelessly transmitted data from the wireless relay transmitter, and an in-vehicle indicator is in communication with the in-vehicle receiver and available to the vehicle operator.