A 29-year-old female green-winged macaw (Ara chloropterus) was presented for acute pelvic limb lameness. On physical examination the bird was found to be mildly dehydrated with paraparesis and proprioceptive deficits of the pelvic limbs. Antemortem diagnostics included a complete blood count and plasma biochemistry panel, fecal Gram and Ziehl-Neelsen acid fast stains, plasma heavy metal concentrations, reverse transcriptase (RT)-PCR testing for West Nile virus (WNV) RNA and PCR testing for Chlamydophila psittaci DNA from choanal/cloacal swabs, and survey radiography. Abnormal findings included a heterophilic leukocytosis, elevated kidney and muscle enzyme values, and a positive RT-PCR result for West Nile viral RNA. Although the bird showed some improvement with supportive care, on the fourth day it began having seizures and was euthanatized. There were no abnormal findings detected on gross necropsy; however, histopathology revealed mononuclear inflammatory cell infiltration in multiple organs including brain, heart, and pancreas. WNV antigen and RNA were detected using immunohistochemistry and RT-PCR in various tissues including brain, pancreas, and spleen. WNV was successfully isolated from brain, pancreas, spleen, and liver. To our knowledge, this is the first report characterizing WNV disease in a green-winged macaw and one of few reports of this disease in a psittacine bird.