Armadillo repeat-containing protein 8 (Armc8) is a key factor to regulate cell membrane adhesion complex through promoting α-catenin degradation. However, its expression and function in human malignant tumors are largely unknown. Here, we present our study investigating Armc8 expression in tumor and non-tumor breast tissues including 45 normal epithelia, 53 lesions of hyperplasia with or without dysplasia, 22 benign tumors, and 92 carcinomas including 28 carcinomas in situ and 64 infiltrating carcinomas using immunohistochemistry (IHC) and Western blotting study. Armc8 expression was detected mainly in the cytoplasm with occasional membrane immunostaining. The positive rate of Armc8 expression in normal breast epithelia (8.9%, four out of 45) was very low. No significant difference was found between Armc8 expression in usual ductal hyperplasia (UDH) (11.1%, two out of 18), benign breast tumors including intraductal papilloma (10.0%, one out of 10) and fibroadenoma (8.3%, one out of 12), and normal breast epithelia (p>0.05). Elevated expression of Armc8 was found in breast epithelia with dysplasia (24.0%, six out of 25) compared to that in normal breast epithelia, UDH, and benign breast tumors (p<0.05). Armc8 expression in breast carcinoma including breast carcinoma in situ (10/28, 35.7%), infiltrating ductal carcinoma (60.7%, 34/56), and infiltrating lobular carcinoma (50.0%, 4/8) was higher than that in normal breast epithelia, UDH, benign breast tumors, and breast epithelia with dysplasia (p<0.05). The highest expression of Armc8 was found in infiltrating breast carcinoma (59.4%, 38/64) compared to all the other breast tissues. Higher Armc8 expression was found to be linked to lymph node metastasis and advanced tumor-node-metastasis (TNM) stages (III+IV) in infiltrating breast carcinoma (p<0.05). We further confirmed Armc8 expression in breast epithelial cell line MCF10A and breast carcinoma cell lines including MCF-7, MDA-MB-231, and ZR751 using Western blotting and immunofluorescent study. These results indicate that the elevated expression of Armc8 may be involved in carcinogenesis including atypia-to-carcinoma progression and cancer development of breast carcinoma.