BACKGROUND: The STarT Back Screening Tool (SBST) was developed to stratify low back pain patients according to their risk of future physical disability so that prognostic subgroups can receive matched treatments in primary care. OBJECTIVE: To measure the construct and discriminative validity of the SBST-Brazil questionnaire. METHOD: A hundred and fifty one patients were recruited to test the construct and discriminative validity comparing the SBST-Brazil to the Brazilian Version of the Oswestry Disability Index (ODI), Roland Morris Disability Questionnaire (RMDQ) and Fear-Avoidance Beliefs Questionnaire-Work (FABQ-W) and Physical Activity (FABQ-PA) subscales at baseline. Spearman's rank-order correlation and area under the curve (AUC) derived from receiver operating curves (ROC) for total scores and psychosocial subscale score of the SBST-Brazil were used for construct and discriminant validity analysis, respectively. RESULTS: The SBST-Brazil total and psychosocial subscale scores had good and moderate correlation with ODI (r=0.61; r=0.56, respectively) and good with RMDQ (r=0.70; r=0.64, respectively). Both scores of the SBST-Brazil total and psychosocial subscale correlated weakly and moderately with the FABQ-PA (r=0.28; r=0.34, respectively) and weakly with the FABQ-W (r=0.18; r=0.20, respectively). The discriminant validity with AUCs for the total and psychosocial subscale scores against reference standard ranged from 0.66 for kinesiophobia to 0.88 for disability. CONCLUSION: The SBST-Brazil showed a moderate to good correlation with the disability tools, but a weak correlation with fear-avoidance beliefs. The results of discriminant validity suggest that SBST-Brazil is able to discriminate low back pain patients with disability and fear-avoidance beliefs.