A deformed rib from a Columbian mammoth (Mammuthus columbi) was found among more than 100 mammoth bones excavated from an alfalfa field at the Tonnemaker Hill Farm in the Frenchman Hills north of Royal City, Grant County, Washington. The mammoth remains were deposited within fine-grained Missoula flood sediments sometime after 16 ka. We performed a detailed examination of the rib, including computerized tomography imaging, and investigated the possible etiology of the deformity. The deformity consists of hypertrophic bone formation localized toward the distal (sternal) end of the rib. The circumference of the lesion is nearly twice that of the medial circumference of the rib diaphysis. This lesion is consistent with an incompletely healed fracture or a fracture nonunion with pseudarthrosis (false joint) formation. Gross examination and computerized tomography scan images of the proximal aspect of the rib, as well as cursory inspection of the other skeletal remains, have yet to reveal definitive evidence of other bony abnormalities. We surmise, therefore, that this fracture was the consequence of blunt force trauma in a healthy animal. To our knowledge, this is the first report of a rib fracture with incomplete healing or false joint formation in a M. columbi individual in the Pacific Northwest. We suspect the incidence of rib fractures among M. columbi was greater than the Pacific Northwest literature suggests and encourage examination of existing collections for evidence of similar findings. The incidence and frequency of such injuries may provide insight into M. columbi behavior.