Abstract Objective The Yale–Brown Obsessive–Compulsive Scale (Y-BOCS) is the most commonly used instrument to assess the clinical severity of obsessive–compulsive symptoms. Treatment determinations are often based on Y-BOCS score thresholds. However, these benchmarks are not empirically based, which may result in non-evidence based treatment decisions. Accordingly, the present study sought to derive empirically-based benchmarks for defining obsessive–compulsive symptom severity. Method Nine hundred fifty-four adult patients with obsessive–compulsive disorder (OCD), recruited through the Brazilian Research Consortium on Obsessive–Compulsive Spectrum Disorders, were evaluated by experienced clinicians using a structured clinical interview, the Y-BOCS, and the Clinical Global Impressions–Severity scale (CGI-Severity). Results Similar to results in treatment-seeking children with OCD, our findings demonstrated convergence between the Y-BOCS and global OCD severity assessed by the CGI-Severity (Nagelkerke R2 = .48). Y-BOCS scores of 0–13 corresponded with ‘mild symptoms’ (CGI-Severity = 0–2), 14–25 with ‘moderate symptoms’ (CGI-Severity = 3), 26–34 with ‘moderate-severe symptoms’ (CGI-Severity = 4) and 35–40 with ‘severe symptoms’ (CGI-Severity = 5–6). Neither age nor ethnicity was associated with Y-BOCS scores, but females demonstrated more severe obsessive–compulsive symptoms than males ( d = .34). Time spent on obsessions/compulsions, interference, distress, resistance, and control were significantly related to global OCD severity although the symptom resistance item pairing demonstrated a less robust relationship relative to other components of the Y-BOCS. Conclusions These data provide empirically-based benchmarks on the Y-BOCS for defining the clinical severity of treatment seeking adults with OCD, which can be used for normative comparisons in the clinic and for future research.