Gastric ulcer is a widespread inflammatory disease with high socio‐economic burden. C‐phycocyanin is one of the active constituents of Spirulina microalgae, and although it is well known for its antioxidant and anti‐inflammatory properties, its protective effects against gastric ulcer have not yet been identified. High‐mobility group box 1 (HMGB1) is a nuclear protein that, once secreted extracellularly, initiates several inflammatory reactions, and it is involved in the pathogenesis of gastric ulcer. The aim of the present study was to investigate the anti‐inflammatory and anti‐ulcerogenic effects of C‐phycocyanin against ethanol‐induced gastric ulcer targeting HMGB1/NLRP3/NF‐κB pathway. Ulcer induction showed increase in HMGB1 expression through activation of nucleotide‐binding domain and leucine‐rich repeat‐containing protein 3 (NLRP3) inflammasome and nuclear factor kappa p65 (NF‐κB p65). Moreover, oxidative stress and inflammatory markers were elevated in the ulcer‐treated group compared to the normal control group. However, pre‐treatment with C‐phycocyanin significantly reduced HMGB1 expression via suppression of NLRP3/NF‐κB, oxidative markers, IL‐1β, tumour necrosis factor‐α (TNF‐α) and ulcer index value. These results were consistent with histopathological and immunohistochemistry examination. Thus, C‐phycocyanin is a potential therapeutic strategy with anti‐inflammatory and anti‐ulcerogenic effects against ethanol‐induced gastric ulcer.