Chromatographic purification of Sigesbeckia glabrescens extracts led to three undescribed ent‐pimarane diterpenoid dimers, named glabreside A—C (1—3). Their structures were confirmed by comprehensive spectroscopic analyses, and the structures of compound 1 and 3 were confirmed by X‐ray crystallography. The results of anti‐inflammatory activity assay showed that glabreside C (3) exhibited the most potent inhibition activity on nitric oxide (NO) production induced by lipopolysaccharide (LPS) in BV2 microglia compared with the other two compounds. Glabreside C also increased the protein expression level of heme oxygenase‐1 (HO‐1), and suppressed inducible nitric oxide synthase (iNOS) and cyclooxygenase 2 (COX2) in LPS‐stimulated BV2 cells. Mechanistically, glabreside C exerts its anti‐inflammatory effect by inhibiting AKT/MAPKs signaling pathway. These findings indicate the vital role of ent‐pimarane diterpenoid dimers in explaining the anti‐inflammatory activity of S. glabrescens and provide important evidence for further development and utilization of Sigesbeckiae Herba. These results also suggest that glabreside C is a potential lead compound for anti‐inflammatory drugs.