AbstractDelphinidin, one of the main anthocyanidins, has potent anti‐cancer properties. In this study, we investigated the effect of delphinidin on 1‐methyl‐1‐nitrosourea (MNU)‐induced breast carcinogenesis on rats and the mechanism of delphinidin via negative regulation of the HOTAIR/microRNA‐34a axis. We found administration of delphinidin could effectively suppress MNU‐induced mammal breast carcinogenesis. Delphinidin downregulated the level of HOTAIR and upregulated miR‐34a in breast carcinogenesis. Western blot analysis confirmed that delphinidin treatment can significantly decrease the expression of β‐catenin, glycogen synthase kinase‐3β (Gsk3β), c‐Myc, cyclin‐D1, and matrix metalloproteinase‐7(MMP‐7) expression in breast cancer cells, and inhibition of miR‐34a significantly reduced the effect of delphinidin on c‐Myc, cyclin‐D1, and MMP‐7. HOTAIR overexpression also blocked the effect of delphinidin on miR‐34a and the Wnt/β‐catenin signaling pathway in MDA‐MB‐231 cells. RNA immunoprecipitation (RIP) assay and chromatin immunoprecipitation (ChIP) assay results showed that delphinidin upregulated miR‐34a by inhibiting HOTAIR, coupled with enhancement of the zeste homolog 2 (EZH2) and histone H3 Lys27 trimethylation (H3K27me3). This study indicated that delphinidin may potentially suppress breast carcinogenesis and exert its anti‐cancer effect through the HOTAIR/miR‐34a axis. These findings provided new evidence for the use of delphinidin in preventing breast carcinogenesis.