Extensive tau-immunoreactive neurons and glial cells associated with chronic traumatic encephalopathy (CTE) have been documented in the brains of some professional athletes and others with a history of repetitive mild traumatic brain injury (r-mTBI). The neuropathology and tau involvement in mTBI have not been extensively studied in animal models, particularly in aged animals. We investigated the effects of single mTBI (s-mTBI) and r-mTBI in 18-month-old hTau mice, which express wild-type human tau isoforms on a null murine tau background (n = 3-5 per group). At this age, hTau mice already demonstrate tau pathology, but there was a significant increase in phospho-tau immunoreactivity in response to r-mTBI, but not to s-mTBI,as determined using multiple phospho-tau-specific antibodies. Repetitive mTBI also resulted in a marked increase in astrocyte/microglia activation notably in the superficial layer of the motor/somatosensory cortex and the corpus callosum. We did not observe the perivascular tau pathology, neuritic threads, or astrocytic tangles that are commonly found in human CTE. The increase in phospho-tau in the r-mTBI mice suggests that this may be a useful model for investigating further the link between mTBI, particularly r-mTBI, and tau pathobiology in CTE and in understanding responses of the aged brain to mTBI.