Previously, we reported that overexpression of Opa (Neisseria gonorrhoeae opacity-associated)-interacting protein 5 (OIP5) caused multi-septa formation and growth defects, both of which are considered cancer-related phenotypes. To evaluate OIP5 as a possible cancer therapeutic target, we examined its expression level in 66 colorectal cancer patients. OIP5 was upregulated about 3.7-fold in tumors and over 2-fold in 58 out of 66 colorectal cancer patients. Knockdown of OIP5 expression by small interfering RNA specific to OIP5 (siOIP5) resulted in growth inhibition of colorectal and gastric cancer cell lines. Growth inhibition of SNU638 by siOIP5 caused an increase in sub-G1 DNA content, as measured by flow cytometry, as well as an apoptotic gene expression profile. These results indicate that knockdown of OIP5 may induce apoptosis in cancer cells. Therefore, we suggest that OIP5 might be a potential cancer therapeutic target, although the mechanisms of OIP5-induced carcinogenesis should be elucidated.