Urokinase plasminogen activator (uPA) and its receptor uPAR promote cancer invasion and metastasis and are emerging therapeutic targets in both human and canine malignancies. While their clinical significance is well-characterized in multiple human tumor types, studies investigating their roles in osteosarcoma are lacking. The objectives of this study were to characterize serum and tissue uPA/uPAR expression in dogs with osteosarcoma and assess the prognostic significance. Serum samples and a tissue microarray of canine appendicular osteosarcoma were analyzed for uPA and uPAR expression by ELISA (n = 49) and immunohistochemistry (n = 38), respectively. Serum uPA activity was also measured by a chromogenic assay (n = 25). Survival analysis was performed by Kaplan-Meier survival analysis, log rank test, and Cox regression analysis. Serum uPA level was significantly higher in dogs with osteosarcoma than clinically healthy control dogs (median 1905 vs 1440 pg/ml, p = 0.008). The majority of canine osteosarcoma tissues expressed uPA (75.9%) or uPAR (77.6%), with 70.7% dual-positivity, indicating autocrine/paracrine activation of the pathway. Survival analysis revealed shorter progression free survival (PFS) in dogs with high serum uPA level in a discovery cohort (n = 29; median PFS 94 vs 266 days, p = 0.003) but not in a validation cohort (n = 23; median PFS 167 vs 490 days, p = 0.16). The difference was significant when both cohorts were combined (n = 49; median PFS 128 vs 266 days, p = 0.003). Serum uPAR and tissue uPA/uPAR levels were not prognostic. In Cox multivariate analysis, high serum uPA level and activity were both associated with poor prognosis, independent of serum ALP, tumor location, and peripheral lymphocyte/monocyte counts. These results indicate high utilization of the uPA pathway and association with disease progression in canine osteosarcoma. Further study involving prospective evaluation to confirm the prognostic significance is warranted. The high prevalence of tissue uPA and uPAR expression suggests the uPA system as a potential therapeutic target in canine osteosarcoma.