BACKGROUND Congenital factor XIII (FXIII) deficiency is an extremely rare bleeding disorder with defects in the F13A1 or F13B genes. Here, we report a case of congenital FXIII deficiency patient who presented with trauma-induced intramuscular hemorrhage accompanied with transient platelet dysfunction with increased endogenous thrombin potential (ETP). METHODS FXIII antigen and activity, F13A1 gene sequencing, and thrombin generation assay were measured. RESULTS The diagnosis of FXIII deficiency was confirmed by a double heterozygous mutation of the F13A1 gene and decreased levels of FXIII antigen and activity. Platelet dysfunction caused by an antiplatelet drug was revealed in both platelet aggregation test and PFA-100. After a bleeding event, the PFA-100 results returned to normal and the thrombin generation assay in patient's plasma showed a higher ETP than normal. CONCLUSIONS This increase in ETP may protect against bleeding and may explain why some patients show only a mild bleeding tendency despite undetectable FXIII activity.