Bernard-Soulier syndrome (BSS) is a rare inherited platelet disorder caused by a defect in any one of the four proteins that make up the platelet receptor called the glycoprotein Ib-IX-V complex (which binds von Willebrand factor), preventing platelets from adhering and agglutinating normally. It is characterized by low platelet counts, giant platelets, and bleeding and easy bruising.

BSS is inherited in an autosomal recessive fashion, affecting both sexes with equal frequency. It is seen in 1 in a million individuals. It is more commonly described in individuals of Northern European descent and those of Japanese descent. The condition is present at birth, although a few cases may not become evident until adulthood.

Patients have a tendency to bleed excessively from injuries and may have recurrent epistaxis and easy bruising; women may experience unusually heavy menstruation.