Abstract Purpose: TGFβ1 and TGFβ receptor 1 (TGFβR1) participate in regulation of the host's immune system and inflammatory responses and may serve as prognostic biomarkers for human papillomavirus (HPV)-associated oropharyngeal squamous cell carcinoma (OPSCC). Experimental Design: This study included 1,013 patients with incident OPSCC, of whom 489 had tumor HPV16 status determined. All patients were genotyped for two functional polymorphisms: TGFβ1 rs1800470 and TGFβR1 rs334348. Univariate and multivariate Cox regression models were performed to evaluate associations between the polymorphisms and overall survival (OS), disease-specific survival (DSS), and disease-free survival (DFS). Results: Patients with TGFβ1 rs1800470 CT or CC genotype had 70%–80% reduced risks of OS, DSS, and DFS compared with patients with TT genotype, and patients with TGFβR1 rs334348 GA or GG genotype had 30%–40% reduced risk of OS, DSS, and DFS compared with patients with AA genotype. Furthermore, among patients with HPV-positive (HPV+) OPSCC, the same patterns were observed but the risk reductions were greater: up to 80%–90% for TGFβ1 rs1800470 CT or CC genotype and 70%–85% for TGFβR1 rs334348 GA or GG genotype. The risk reductions were still greater (up to 17 to 25 times reduced) for patients with both TGFβ1 rs1800470 CT or CC genotype and TGFβR1 rs334348 GA or GG genotype compared with patients with both TGFβ1 rs1800470 TT genotype and TGFβR1 rs334348 AA genotype among patients with HPV+ OPSCC. Conclusions: Our findings indicate that TGFβ1 rs1800470 and TGFβR1 rs334348 may individually or jointly modify risks of death and recurrence in patients with OPSCC, particularly those with HPV+ OPSCC undergoing definitive radiotherapy, and may serve as prognostic biomarkers, which could lead to better personalized treatment and improved prognosis.