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Acute lung injury (ALI) is a respiratory disease that is associated with neutrophil activation. Neutrophil elastase released from activated neutrophils has been reported to cause lung tissue damage and contribute to the inflammatory response in ALI. In this study, aqueous extracts of Kochia scoparia fruits, a traditional botanical medicine and food garnish in Asia, showed a selective inhibitory effect on human neutrophil elastase (IC50, 79.53 ± 4.26 μg mL−1). Using bioactivity-guided fractionation, the polysaccharide fraction (KSWP, molecular weight > 300 kDa) was identified, and its protective effects against ALI were investigated. The results showed that KSWP protected mice from lipopolysaccharide-induced ALI by reducing elastase activity, neutrophil infiltration, pro-inflammatory cytokines, and neutrophil NET formation. In addition, KSWP showed high safety parameters in toxicity tests. These results conclusively demonstrate the high potency of KSWP as a component of functional foods or botanical products in the prevention of ALI.