Objective: To identify platelet hypofunction and its associations in severely injured trauma patients presenting with haemorrhage. Design: Planned sub-study of data collected from the FEISTY trial; an Australian multicentre, randomised controlled pilot trial investigating early fibrinogen replacement in severely injured trauma patients. Setting: Four major trauma centres in Queensland, Australia. Participants: Adult trauma patients (age ≥18 years) presenting with clinically significant haemorrhage or potential for significant transfusion requirements. Main outcome measures: Platelet function parameters arachidonic acid (ASPI), adenosine diphosphate (ADP), and thrombin receptor-activating peptide (TRAP) assessed via Multiplate® analysis, rotational thromboelastometry (ROTEM®) parameters EXTEM, FIBTEM, and PLTEM (EXTEM – FIBTEM), transfusion requirements, and clinical outcomes. Results: Significant platelet hypofunction was detected in this cohort of severely injured trauma patients at time of presentation, with 70 % of patients having hypofunction in at least one Multiplate® channel. The median ASPI area under the curve and 95 % confidence interval were below the lower reference range, indicating this population had reduced platelet function. In patients with deranged platelet function, significantly lower platelet count (p ≤ 0.001), EXTEM amplitude at five minutes (A5) and maximum clot firmness (MCF) (p = 0.001, p ≤ 0.001), and PLTEM A5 and MCF (p = 0.005, p = 0.003) were identified compared to patients with normal platelet function. A significant improvement in platelet function parameters was not observed following platelet transfusion. Conclusion: Platelet hypofunction is common in severely injured trauma patients. This was true both before and after platelet transfusion, suggesting trauma precipitates alteration of the vascular circulating milieu in a way that impairs platelet function. Characterisation of this change might lead to targeted interventions to improve haemostasis.