Adenoviral vector vaccines have been epidemiologically linked to venous-predominant thrombotic events, yet the mechanistic contribution of RBCs remains largely unexplored. Here we provide experimental evidence that RBCs are active drivers—not passive bystanders—of venous thrombogenesis under adenoviral vector exposure. Using an Ad5 vector rat model within a predefined sub-hemolytic window (<10% hemolysis), we identified a coordinated procoagulant cascade: surface phosphatidylserine (PS) exposure increased significantly at 4 h post-injection, RBC morphology progressively transitioned from discocytes through echinocytes to spherocytes, and PS-enriched microvesicle shedding was markedly elevated. These structural changes drove dose-dependent whole-blood thrombin generation. In vivo, an inferior vena cava model confirmed a pronounced, dose-responsive increase in thrombus burden. Critically, this entire prothrombotic sequence unfolded without overt hemolysis, implicating RBC membrane remodeling—rather than lysis—as the principal mechanism. These findings challenge the platelet/PF4-centric framework of vaccine-associated thrombosis and establish an RBC-aware paradigm particularly relevant to low-shear venous environments. Incorporating RBC-focused endpoints—PS exposure, morphology indices, microvesicle counts, and thrombin generation—into preclinical and clinical safety evaluation may improve risk stratification and guide next-generation adenoviral vector design.