Epilobium angustifolium (EA) is acknowledged in China for its dual use as both a medicinal herb and an edible plant. The pharmacological effects of EA mainly attribute to its high concentrations in its secondary metabolite: Oenothein B (OB). OB has shown anti-tumour activity in various cancer cell lines. However, its efficacy and immunopharmacological mechanisms in the treatment of melanoma remain largely unknown. Therefore, we evaluated the anti-tumour activity of OB in B16-F10 cells both in vitro and in vivo. The results revealed that OB suppressed the growth, migration, invasion, and cell cycle of B16-F10 cells in a dose-dependent manner while promoting apoptosis. OB inhibited B16-F10 cell growth by altering the tumour immune microenvironment, in which numbers of macrophages and T cells were elevated. Subsequent research showed that OB inhibited mRNA synthesis of hypoxia-inducible factor (HIF)-1α via PI3K/AKT/NF-κB pathway, which led to the reduced expression of CD47 and PD-L1. In addition, OB enhanced macrophage phagocytosis by promoting macrophage polarisation from M2 to M1 via the PI3K/AKT pathway, and it also enhanced the tumour-killing activity of cytotoxic T lymphocytes (CTLs). In summary, our findings uncover the hitherto unidentified mechanism in which OB suppresses CD47 and PD-L1 in B16-F10 cells, and enhances phagocytosis of these cells by macrophages and killing of these cells by T cells. These results suggest that OB holds promise as a potent immunotherapeutic agent for the treatment of melanoma cancer.