Gilbert's syndrome (GS) individuals have mildly elevated serum unconjugated bilirubin (UCB) concentrations and are protected against the development of cardiovascular diseases (CVD). Although UCB has antioxidant properties, which could delay atherosclerotic plaque development, evidence suggests UCB might also affect haemostasis, subsequently influencing thrombus formation after atherosclerotic plaque rupture. The aim of this study was to reveal the in-vitro effect of UCB on platelet function and haemostatic factors at physiologically relevant concentrations seen in GS. Blood samples were collected from 16 healthy volunteers (mean age 25 ± 5) for full blood examination. A final concentration of approximately 35 ± 4.0 µmol/L of UCB was obtained by adding 1.25 µL of UCB stock solution to 250 µL of sample, to study its effect on platelet aggregation, coagulation and lipid profile. Collagen induced platelet aggregation was significantly inhibited in platelet rich plasma treated with UCB. Coagulation and lipid profile did not change by the in-vitro addition of UCB. These data are the first to show that mildly (but physiologically) elevated UCB inhibits platelet activity in plasma via a mechanism specifically related to collagen induced platelet activation. These findings support a novel mechanism which might further explain protection from CVD by mildly elevated levels of UCB, thus reducing the risk of thrombus formation by inhibition of collagen-induced platelet aggregation.