COVID-19 is characterized by diffuse alveolar damage (COVID-19-DAD). The COVID-19-associated hemostatic alterations manifested by venous and arterial thrombosis deserve special attention. The issue is especially urgent for patients with diabetes mellitus. An observational study aimed to assess the role of polymorphisms of the genes ITGB3, ITGA2, and GP1BA in aggregation alteration in patients having COVID-19-DAD combined with type 2 diabetes mellitus (T2D). The control group included healthy volunteers (group 1; n = 22). Patients with COVID-19-DAD were divided into two group based on the fact of having or not having T2D: group 2 with no T2D (n = 52) and group 3 with T2D (n = 56). Platelet aggregation was assessed with the ALAT-2 laser platelet aggregation analyzer; adenosine diphosphate (ADP) at a concentration of 2.5 µg/mL, collagen 2.0 µg/mL, adrenaline 5 µg/mL, and ristomycin 7.5 mg/mL were used as aggregation inducers. In patients with СOVID-19-DAD and T2D, the rate of aggregate formation with the ADP induction was higher in carriers of the Т/С and С/С variants compared to carriers of the Т/Т variant – by 14 and 23% based on the median (р < 0.05). Polymorphisms were determined by polymerase chain reaction in buccal epithelial scrapings. In the group of patients with СOVID-19-DAD having no T2D, depending on the ITGA2 gene rs1126643 polymorphism the collagen-induced platelet aggregation is accelerated with the С/Т and Т/Т variants compared to the С/С variant — by 55 and 49%, respectively (р < 0.05); among individuals with СOVID-19-DAD and T2D, carriers of the Т/Т variant show the rate of the collagen-induced platelet aggregation 30% higher compared to carriers of the С/С variant (р < 0.05). Conclusion: in patients with COVID-19-DAD and T2D, the presence of the mutant allele Т of the ITGA2 gene rs1126643 polymorphism and allele С of the ITGB3 gene rs5918 polymorphism increases the rate of the collagen- and ADP-induced platelet aggregation.