Sulfated galactans are polysaccharides with heterogeneous structures that frequently show anticoagulant activity. Their anticoagulant mechanisms are complex and distinct from those observed for heparin. Sulfated galactans act through a combination of effects involving serpin-dependent and serpin-independent mechanisms. Interestingly, these polymers can also induce blood coagulation due to activation of factor XII (FXII).The structure of a complex sulfated galactan from the red alga Acanthophora muscoides was characterized by solution nuclear magnetic resonance. This polysaccharide and another previously characterized algal sulfated galactan from Botryocladia occidentalis were each used in in vitro and in vivo anticoagulant and antithrombotic assays to understand the possible structural determinants of their functional effects.The serpin-dependent anticoagulant effects and FXII-related procoagulant effects of the sulfated galactans decreased in parallel with the molecular size. The serpin-independent anticoagulation also correlated with the chemical structure of the sulfated galactans. The sulfated galactan from A. muscoides, which showed mostly serpin-independent anticoagulant activity and reduced activation of FXII, drastically reduced arterial thrombus formation. However, the sulfated galactans produced opposite effects on venous thrombosis; this difference appears to result from the tenuous balance between the various effects on coagulation, including serpin-dependent and serpin-independent anticoagulation and FXIIa-dependent procoagulation. This study of novel sulfated polysaccharides with distinct effects on coagulation and thrombosis helps to establish the minimal structural-function relationship required for the development of antithrombotic drugs.