There is a pressing need to identify novel antiplatelet agents, an alternative to acetylsalicylic acid and thienopyridines, to broaden the prevention of cardiovascular events, the leading cause of global morbidity and mortality. Invertebrate coelomocytes structurally and functionally resemble the thrombocyte-like cells of vertebrates; therefore, the coelomic fluid in which they are suspended may contain agents controlling their clumping abilities. However, whether coelomocytes-free coelomic fluid may also affect human platelet activities was not a subject of any study. This study aimed to screen the in vitro antiplatelet and anticoagulant activities of the polysaccharide-protein complex from Dendrobaena veneta coelomic fluid (25–100 µg/mL) (PPC-DV). All tested fluid concentrations induced significant (42.4–52.5%) inhibition of adenosine-5′-diphosphate (ADP)-induced aggregation of human platelets at a level comparable to that of 140 µmol/L acetylsalicylic acid. Its relevant antiplatelet effect (27.2–45.9%) was also evidenced in the thrombin receptor-activating peptide-6 (TRAP-6) assay. Moreover, 50 and 100 µg/mL of PPC-DV inhibited arachidonic acid-inducible aggregation. No coagulopathic or cytotoxic effects of PPC-DV were observed. The study indicates that PPC-DV, at a concentration of at least 50 µg/mL, exerts a favorable antiplatelet effect by targeting at least three pathways (P2Y₁₂ receptor, cyclooxygenase-1, and protease-activated receptor-1), justifying further experimental and clinical investigations on its use in cardiovascular disease prevention.