<p>Rituximab, an anti-CD20 monoclonal antibody, is a cornerstone of therapy for diffuse large B-cell lymphoma (DLBCL), yet a substantial proportion of patients develop treatment failure. While loss of CD20 expression has been considered a major mechanism of resistance, additional mechanisms limiting effective antibody binding remain incompletely understood. We hypothesized that circulating CD20-positive large extracellular vesicles (EV^large) act as antigen sinks that sequester rituximab and attenuate its therapeutic activity. Circulating CD20-positive EV^large were quantified from archived serum samples of newly diagnosed DLBCL patients treated with R-CHOP. The prognostic impact of EV^large levels was evaluated in a training cohort (<i>n</i> = 26) and an independent validation cohort (<i>n</i> = 90). Functional effects of tumor-derived EV^large were assessed using OCI-Ly1 and U2932 lymphoma cell models and T cell cytotoxicity assays. Rituximab binding to tumor-derived EV^large was visualized using light-field 4D microscopy. CD20 positivity was consistently higher in EV^large than in EV^small across lymphoma cell lines. Tumor-derived CD20-positive EV^large enhanced lymphoma cell proliferation and attenuated rituximab-mediated growth inhibition. EV^large also impaired rituximab-mediated cytotoxicity in T cell co-culture assays. Light-field 4D microscopy demonstrated binding of rituximab to EV^large near the tumor cell surface with reduced antibody binding to lymphoma cells, supporting an antigen-decoy mechanism. Clinically, high pre-treatment levels of circulating CD20-positive EV^large were associated with significantly inferior disease-specific and overall survival in both training and validation cohorts and remained independently prognostic after adjustment for International Prognostic Index risk. In conclusion, tumor-derived CD20-positive EV^large represent a novel mechanism of rituximab resistance by reducing effective antibody exposure and impairing immune-mediated cytotoxicity. Circulating CD20-positive EV^large provide a biologically informative biomarker that predicts clinical outcomes independently of established prognostic factors and may guide optimization of antibody-based therapy in DLBCL.</p>