The WW domain-containing oxidoreductase (WWOX) is a tumor suppressor in a variety of cancers, including breast cancer. Reduced WWOX expression is associated with the basal-like subtype and a relatively poor disease-free survival rate among breast cancer patients. Though several WWOX partners have been identified, the functional mechanisms of WWOX’s role in cancers have not been fully addressed to date. In the current study, we found WWOX suppresses expression of KLF5—an oncogenic transcription factor—at protein level, and suppresses cancer cell proliferation in both bladder and breast cancer cell lines. Furthermore, we demonstrated that WWOX physically interacts with KLF5 via the former’s WW domains and the latter’s PY motifs. Interestingly, we found the expression of WWOX negatively correlates with KLF5 expression in a panel of breast cancer cell lines. Taken together, we conjecture that WWOX may suppress cancer cell proliferation partially by reducing the expression of KLF5.