We describe a case of posttraumatic lateral tibial plateau fracture that was not demonstrated by conventional radiography. Knee joint ultrasound showed mild lateral meniscal protrusion suggestive of an intra-articular injury, which was confirmed at magnetic resonance imaging to represent an occult lateral tibial plateau fracture. A high index of suspicion for occult fractures should be maintained in posttraumatic patients with ultrasound findings of meniscal protrusion, in spite of apparently normal X-ray findings.