Avulsion injuries of the tibial tuberosity are uncommon fractures that are seen most frequently in adolescents. Watson-Jones classified these injuries into three types, but this classification does not account for fractures of the tibial tuberosity that extend into the posterior cortex, nor has such a fracture configuration been described in the literature. The present case of a 16-year-old boy indicates that the Watson-Jones classification should be expanded to include this fracture configuration (Type IV).