Synurus deltoides (Aiton) Nakai, belonging to the Compositae family, is an edible plant widely distributed in Northeast Asia. In this study, we examined the mechanisms underlying the immunomodulative effects of the ethanol extract of S. deltoides (SDE). The SDE extract strongly down-regulated the mRNA expression of the inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), and tumour necrosis factor (TNF)-α, thereby inhibiting the production of nitric oxide (NO), prostaglandin E₂ (PGE₂), and TNF-α in the lipopolysaccharide (LPS)-stimulated RAW 264.7 cells. Furthermore, SDE also suppressed the nuclear translocation of the activation protein (AP)-1 and the nuclear factor-κB (NF-κB), and simultaneously decreased the phosphorylation of extracellular signal-regulated protein kinases (ERK), p38, and Akt. In agreement with the in vitro observations, the orally administered SDE ameliorated the acute inflammatory symptoms in the arachidonic acid-induced ear edema and the EtOH/HCl-induced gastritis in mice. Therefore, S. deltoides have a potential anti-inflammatory capacity in vitro and in vivo , suggesting the potential therapeutic use in the inflammation-associated disorders.