Fibrinogen γA/γ′ results from alternative splicing of mRNA. This variant, which constitutes approximately 8% to 15% of plasma fibrinogen, contains FXIII and thrombin binding sites. Our objective was to investigate whether γA/γ′ differs in fibrin formation and structure from the more common variant γA/γA. Both variants were separated and purified by anion-exchange chromatography. Fibrin formation and clot structure of the variants and unfractionated fibrinogen were investigated by turbidity and scanning electron microscopy (SEM). Thrombin cleavage of fibrinopeptides was analyzed by high-performance liquid chromatography (HPLC). Turbidity analysis showed significantly altered polymerization rates and overall fiber thickness in γA/γ′ clots compared with γA/γA and unfractionated fibrinogen. This finding was consistent with a range of thrombin concentrations. HPLC demonstrated reduced rates of fibrinopeptide B (FpB) release from γA/γ′ fibrinogen compared with γA/γA. Delayed FpB release was associated with delayed lateral aggregation of protofibrils and significant differences were found on SEM, with γA/γ′ clots consisting of smaller diameter fibers and increased numbers of branch points compared with both γA/γA and unfractionated fibrinogen. These results demonstrate that the γA/γ′ splice variant of fibrinogen directly alters fibrin formation and structure, which may help to explain the increased thrombotic risk associated with this variant.