Jamestown Canyon virus (JCV) is a mosquito-borne zoonotic pathogen belonging to the California serogroup of bunyaviruses. Although JCV is widely distributed throughout temperate North America, reports of human JCV infection in the United States are rare. This is the first report of human JCV infection detected in Montana, one of only 15 cases reported in the United States since 2004, when JCV became reportable. On May 26, 2009, a man aged 51 years with no travel history outside of Montana went to a local emergency department immediately following onset of fever, severe frontal headache, dizziness, left-sided numbness, and tingling. His blood pressure was elevated. Stroke was ruled out, oxygen was administered, medication was prescribed for hypertension, and the patient was sent home. One week later, the patient visited his primary-care physician complaining of continued neurologic symptoms consistent with acute febrile encephalitis and recent mosquito bites. Although West Nile virus (WNV) disease was diagnosed based on detection of WNV-immunoglobulin M (IgM) and G (IgG) antibodies, subsequent testing indicated that the WNV antibodies were from a past infection and that his illness was caused by JCV. The final diagnosis of JCV infection was based on positive JCV-specific IgM enzyme-linked immunosorbent assay (ELISA) results and a fourfold rise in paired sample JCV plaque reduction neutralization test (PRNT) titers. This finding represents a previously unrecognized risk for JCV infection in Montana; clinicians should consider JCV infection when assessing patients for suspected arboviral infections.