Transoral robotic surgery (TORS) is an established treatment for early-stage oropharyngeal squamous cell carcinoma (OPSCC). Tumours involving the glossotonsillar sulcus (GTS) are hypothesised to confer worse oncological outcomes compared to those situated in the rest of the oropharynx, with a paucity of evidence to support this theory. We present a single-centre prospective observational cohort study of consecutive adults undergoing TORS for pT1–2 OPSCC between 2016 and 2023. Tumours were classified as GTS-involving or non-GTS. Demographic, clinicopathological and treatment data were collected, including margin status and adverse histological features. Overall survival (OS) and disease-free survival (DFS) were analysed using Kaplan–Meier methods with log-rank testing, and Cox regression was used to identify independent predictors of OS. Of the 153 patients (155 tumours), 52 cancers (34%) involved the GTS. Baseline age, sex, T and N stage, and use of adjuvant therapy were similar between groups. GTS tumours were significantly less likely to achieve clear margins (46% vs 68%, p=0.0078), with close or involved margins more often driven by the deep margin (77% vs 50%, p=0.044). Perineural invasion was more frequent in GTS cancers (27% vs 11%, p=0.033), whereas rates of lymphovascular invasion, extranodal extension, and level I nodal metastases were comparable. There was a non-significant trend towards a higher proportion of p16-negative disease and greater use of adjuvant chemoradiotherapy in the GTS group. Five-year OS was significantly worse with GTS involvement (66% vs 91%, p=0.0036), and on multivariable analysis GTS involvement and p16 status, but not T stage, independently predicted OS. DFS showed a similar but non-significant trend to inferiority (p=0.10), with higher recurrence rates in GTS cancers (17% vs 7%, p=0.068). These significant findings in a contemporary TORS-treated OPSCC cohort identify glossotonsillar sulcus involvement to be a high-risk subsite associated with adverse pathological features.