Carbonic anhydrase IX (CA9) is reportedly overexpressed in several types of carcinomas, but little is known about the expression pattern of CA9 in oral cavity cancer and the corresponding normal tissues. We aimed to assess the prevalence of CA9 expression and its prognostic implications in patients with oral cavity squamous cell carcinoma (SCC). Immunohistochemical staining with anti-CA9 antibody was performed in 117 oral SCC samples. Clinicopathologic factors were correlated with the results of CA9 expression. CA9 expression was restricted to tumor cells and did not appear in the corresponding normal tissue. Among 117 samples, 68 (58.1%) tumors displayed CA9 overexpression. CA9 expression was significantly associated with postoperative recurrence (P = .05) and poor overall survival (P = .02). CA9 expression was also associated with male sex, lymph node metastasis, and smoking history, but these correlations did not reach statistical significance. In conclusion, CA9 expression was a frequent and tumor-specific event in oral SCC. CA9 demonstrated significant associations with disease recurrence and poor clinical outcome and shows potential as a prognostic factor for oral SCC.