The serum phosphorylated neurofilament heavy subunit (pNF-H) is a nervous system-specific protein that is released from damaged neural tissue after traumatic brain injury (TBI). The aim of this study was to elucidate the usefulness of serum pNF-H as a predictive marker for the outcome of patients after TBI. Patients with TBI (Glasgow Coma Scale score of 13 or less on admission) were included. Patients who were younger than age 18, dependent on others for daily activities before injury, pregnant, or who were not likely to survive for more than 24 h after injury were excluded. The outcome was assessed using the Glasgow Outcome Scale at 6 months after injury. Blood was collected from subjects (n = 32), and the serum pNF-H value was assessed at 24 and 72 h after TBI. The optimal cutoff value and usefulness of the serum pNF-H value for predicting the long-term outcome were investigated. We found that the serum pNF-H value at 24 h after injury was a good predictive marker of death at 6 months (p < 0.001) after injury. The optimal cutoff value was 240 pg/mL, and the area under the curve in the receiver operating characteristic analysis was 0.930. The serum pNF-H value at 72 h after injury was correlated with an unfavorable outcome (vegetative state or death) at 6 months (p < 0.01) with a cutoff value of 80 pg/mL. Collectively, the results of this study indicate that the serum pNF-H value is a useful predictive marker for patient outcome after TBI.