Microvascular hyperpermeability is a leading mechanism responsible for occurrence of edema in remote organs and tissues in patients with burn injury. Accumulated evidence has shown that exosomes can be transported into target cells, where they are capable of regulating biological functions and physiology. Of exosomal proteins contributing to enhanced inflammation and vascular permeability, S100 calcium binding protein A9 (S100A9) has received increasing attention. Here we hypothesized that S100A9-containing serum exosomes of patients with burn injury contribute to pathogenesis of hyperpermeability of microvascular structure in lung by transferring signaling molecules into it and activating downstream signaling pathways, ultimately leading to disruption of the tight junctions (TJs) and endothelial barrier. A use of enzyme-linked immunosorbent assay revealed that total serum concentrations of S100A9 were significantly augmented in burn injury patients in comparison to normal controls. With use of human pulmonary microvascular endothelial cells (HPMECs) as an in vitro model, we found that patients' serum exosomes were effectively internalized by HPMECs. We further found that serum exosomes of stage II/II burn patients inhibited zonula occludens (ZO-1) and occludin protein levels, which are essential for TJs integrity and endothelial barrier function, but activated p38 MAPK signaling pathway in HPMECs. As expected, such exosomes-mediated effects on HPMECs were reversed by a simultaneous treatment of anti-S100A9 neutralizing antibody. Finally, we found that a recombinant human S100A9 treatment led to inhibition of expression of occludin and ZO-1 but an activation of p38 signaling in HPMECs, and that such effects were reversed when p38 activity was repressed, implying that S100A9 may stimulate p38 activity to inhibit ZO-1 and occludin in HPMECs. Collectively, these data suggest that S100A9-containing serum exosomes may play a critical role in contributing to pulmonary microvascular hyperpermeability, thus supporting that blocking exosomes' access to HPMECs could hold a promise strategy for treatment of lung edema resulting from burn injuries.