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The activation of Wnt/β‐catenin signalling has an important function in gastrointestinal tumorigenesis. It has been suggested that the promotion of Wnt/β‐catenin activity beyond the threshold is important for carcinogenesis. We herein investigated the role of macrophages in the promotion of Wnt/β‐catenin activity in gastric tumorigenesis. We found β‐catenin nuclear accumulation in macrophage‐infiltrated dysplastic mucosa of the K19‐Wnt1 mouse stomach. Moreover, macrophage depletion in Apc^Δ716 mice resulted in the suppression of intestinal tumorigenesis. These results suggested the role of macrophages in the activation of Wnt/β‐catenin signalling, which thus leads to tumour development. Importantly, the conditioned medium of activated macrophages promoted Wnt/β‐catenin signalling in gastric cancer cells, which was suppressed by the inhibition of tumour necrosis factor (TNF)‐α. Furthermore, treatment …