Few studies have investigated the diagnostic performance of fluorine‐18‐fluorodeoxyglucose (18F‐FDG) positron emission tomography/computed tomography (PET/CT) for staging veterinary patients with appendicular osteosarcoma. The purpose of this study was to evaluate the efficacy of 18F‐FDG‐PET/CT compared to whole‐body CT (WBCT) for staging canine patients with appendicular osteosarcoma. The 18F‐FDG‐PET/CT imaging studies of 66 dogs with appendicular osteosarcoma were anonymized and separated into two detached studies (one with whole body pre‐ and post‐contrast CT images and the other with the whole body pre‐ and post‐contrast CT images with the associated 18F‐FDG‐PET overlay). Image assessment was performed retrospectively by five board‐certified veterinary radiologists. The radiologists were instructed to assign a predefined categorical score (1–4) to each pre‐designated anatomic region based on a devised lesional scoring system. A score of 1 was normal, 2 abnormal but not neoplastic, 3 abnormal and concerning for neoplasia, and 4 abnormal, most likely neoplastic. Overall, the likelihood of detection of ‘3 or 4’ was found to be significantly higher with 18F‐FDG PET/CT when compared to WBCT after adjusting for the effect of evaluator and the subject. Most significantly, 13 osseous lesions concerning for metastasis (scored 3–4) were identified in 10/66 dogs by at least one reviewer on 18F‐FDG PET/CT, which were not identified by any reviewer on WBCT. Additionally, four comorbid neoplastic lesions were identified with 18F‐FDG PET/CT and not with WBCT. The results of this study suggest that 18F‐FDG PET/CT is more efficacious in detecting metastatic and comorbid neoplastic lesions compared to WBCT in dogs with appendicular osteosarcoma.