Cancer susceptibility candidate 2 (CASC2), a long non-coding RNA (lncRNA), has been demonstrated to be a tumor suppressor in several types of cancer. However, the role and mechanism of CASC2 in breast cancer (BC) have not been investigated. In the present study, the expression and functions of CASC2 in BC were investigated. The expression of CASC2 was significantly decreased in BC tissues and cells compared with adjacent normal tissues and mammary epithelial cells, respectively. CASC2 overexpression inhibited the viability, migration and invasion, and elevated apoptosis of BC cells. In addition, CASC2 acted as a competing endogenous RNA for hsa-microRNA (miR)-96-5p and regulated the expression of its target gene, synoviolin (SYVN1). In miR-96-5p-overexpressed MDA-MB-231 cells, cell viability, migration and invasion was increased, and cell apoptosis was decreased, which was reversed by the upregulation of SYVN1. Taken together, the present study data indicated that decreased SYVN1 expression was a tumor suppressor, which inhibited the growth and metastasis of BC through the miR-96-5p/SYVN1 axis.