Tumor-derived extracellular vesicles (tEVs) are important signals in tumor–host cell communication, yet it remains unclear how endogenously produced tEVs affect the host in different areas of the body. We combined imaging and genetic analysis to track melanoma-derived vesicles at organismal, cellular, and molecular scales to show that endogenous tEVs efficiently disseminate via lymphatics and preferentially bind subcapsular sinus (SCS) CD169⁺ macrophages in tumor-draining lymph nodes (tdLNs) in mice and humans. The CD169⁺ macrophage layer physically blocks tEV dissemination but is undermined during tumor progression and by therapeutic agents. A disrupted SCS macrophage barrier enables tEVs to enter the lymph node cortex, interact with B cells, and foster tumor-promoting humoral immunity. Thus, CD169⁺ macrophages may act as tumor suppressors by containing tEV spread and ensuing cancer-enhancing immunity.