AbstractZinc (Zn²⁺) can modulate platelet and coagulation activation pathways, including fibrin formation. Here, we studied the (patho)physiological consequences of abnormal platelet Zn²⁺ storage and release. To visualize Zn²⁺ storage in human and mouse platelets, the Zn²⁺ specific fluorescent dye FluoZin3 was used. In resting platelets, the dye transiently accumulated into distinct cytosolic puncta, which were lost upon platelet activation. Platelets isolated from Unc13d^−/− mice, characterized by combined defects of α/δ granular release, showed a markedly impaired Zn²⁺ release upon activation. Platelets from Nbeal2^−/− mice mimicking Gray platelet syndrome (GPS), characterized by primarily loss of the α-granule content, had strongly reduced Zn²⁺ levels, which was also confirmed in primary megakaryocytes. In human platelets isolated from patients with GPS, Hermansky-Pudlak Syndrome (HPS) and Storage Pool Disease (SPD) altered Zn²⁺ homeostasis was detected. In turbidity and flow based assays, platelet-dependent fibrin formation was impaired in both Nbeal2^−/− and Unc13d^−/− mice, and the impairment could be partially restored by extracellular Zn²⁺. Altogether, we conclude that the release of ionic Zn²⁺ store from secretory granules upon platelet activation contributes to the procoagulant role of Zn²⁺ in platelet-dependent fibrin formation.