AbstractBACKGROUNDA polysaccharide was purified in this study, which was acquired from the fermentation broth of Dendrobium officinale Kimura et Migo. We aimed to investigate the structural features and bioactivity of this polysaccharide.RESULTSThe polysaccharide was purified and the main polysaccharide fraction (i.e., DOP‐1) was obtained. High‐performance gel permeation chromatography (HPGPC) revealed that the molecular weight of DOP‐1 was 447.48 kDa. Galactose, glucose and mannose were found to be present in DOP‐1 via monosaccharide composition analysis, at a ratio of 1:1.79:6.71. Methylation and nuclear magnetic resonance spectroscopic analysis indicated that the backbone of DOP‐1 was →4)‐α‐d‐Glcp‐(1 → 4)‐α‐d‐Manp‐(1 → 4)‐α‐d‐Manp‐(1 → 4,6)‐α‐d‐Manp‐(1→, and its repeating units were also preliminarily established. In vitro tests proved that DOP‐1 not only protects RAW264.7 macrophages from the cytotoxic effect induced by lipopolysaccharide (LPS), but also inhibits cytokines (i.e., interleukin‐6 and tumour necrosis factor‐α) induced by LPS. DOP‐1 demonstrated good scavenging activity in vitro toward 1,1‐diphenyl‐2‐picrylhydrazyl and hydroxyl radicals, as well as good metal chelating activity. Therefore, DOP‐1 has potential antioxidant applications.CONCLUSIONThe structural characteristics of DOP‐1 support its favourable biological activities and lay a strong foundation for further exploration of its structure–activity relationships and activity development, providing experimental data for the development and utilisation of fermentation broth of D. officinale. © 2021 Society of Chemical Industry.