Abstract Background Adenoid cystic carcinoma and primary squamous cell carcinoma are both rare breast neoplasms, which possess drastically different morphological and molecular features as well as distinguishing clinical behaviors and prognosis. Case presentation In this report, we described a rare case in which concurrent adenoid cystic carcinoma and keratinizing squamous cell carcinoma were diagnosed in contralateral breasts in an 85 year-old female patient. The patient had a history of adenoid cystic carcinoma diagnosed 11 years ago, which was treated by partial mastectomy followed by whole breast radiation. The recurrent carcinoma on the same side of the breast was small in size but appeared to involve an intraductal papilloma. Also, a newly occurred large cystic mass was identified on the contralateral breast, which histologically presented as a keratinizing squamous cell carcinoma with no glandular differentiation. No in situ or invasive carcinoma was identified in the overlying skin of the lesion, and no malignancy in a second site was found by PET-CT. Therefore, this lesion was mostly likely a primary squamous cell carcinoma of the breast. Conclusion The concurrence of two such rare neoplasms was likely an incidental finding or was therapy-related. However, more mechanistic studies are needed in order to understand whether predisposing genetic alterations exist in this rare case. Besides, cases of both breast adenoid cystic carcinoma and carcinoma with predominant squamous differentiation diagnosed in our institution were reviewed, which help to better characterize their clinicopathological features.