Background Diagnosis of heparin‐induced thrombocytopenia (HIT) requires pretest probability assessment and dedicated laboratory assays. Objective To develop a pretest score for HIT. Design Observational; analysis of prospectively collected data of hospitalized patients suspected with HIT (ClinicalTrials.gov NCT00748839). Setting Thirty‐one tertiary hospitals in France, Switzerland, and Belgium. Patients Patients tested for HIT antibodies (2280 evaluable), randomly allocated to derivation and validation cohorts. Measurements Independent adjudicators diagnosed HIT based on the prospectively collected data and serotonin release assay results. Results Heparin‐induced thrombocytopenia was diagnosed in 234 (14.7%) and 99 (14.5%) patients in the two cohorts. Eight features were associated with HIT (in brackets, points assigned for score calculation of the score): unfractionated heparin (1); therapeutic‐dose heparin (1); cardiopulmonary bypass (cardiac surgery) (2); major trauma (3); 5‐ to 21‐day interval from anticoagulation initiation to suspicion of HIT (4); ≥40% decrease in platelet count over ≤6 days (3); thrombotic event, arterial (3) or venous (3). The C‐statistic was 0.79 (95% CI, 0.76–0.82). In the validation cohort, the area under the receiver operating characteristic curve was 0.77 (95% CI, 0.74–0.80). Three groups of scores were defined; HIT prevalence reached almost 30% in the high‐probability group. Limitation The performance of the score may depend on settings and practices. Conclusion The objective, easy‐to‐collect, clinical features of HIT we evidenced were incorporated into a pretest score, which may guide clinical decisions regarding diagnostic testing and anticoagulation.