Purpose: SET is an endogenous PP2A inhibitor that might represent a novel molecular target for antitumor therapy. The aim of this study was to evaluate the molecular effects of SET deregulation and its potential clinical significance in metastatic colorectal cancer (CRC). Experimental Design: We studied the biological effects of SET on cell growth, colonosphere formation, caspase activity, PP2A activation status and sensitivity to oxaliplatin and FTY720 treatments. Moreover, we analyzed SET expression by immunostaining in 242 patients with metastatic CRC. Results: SET deregulation promotes cell growth and colonosphere formation, and inhibits PP2A then impairing its antitumor effects. Moreover, SET reduces sensitivity to oxaliplatin in CRC cell lines, which is restored after FTY720 treatment. SET overexpression was detected in 24.8% (60/242) of metastatic CRC patients and determined significantly shorter overall (8.6 vs. 27 months; P < 0.001) and progression-free survival (7.1 vs. 13.7 months; P < 0.001), and poor response to oxaliplatin-based chemotherapy (P = 0.004). Interestingly, its prognostic value was particularly evident in patients younger than 70 years and in those harboring KRAS mutations. Conclusions: SET overexpression is a frequent event in metastatic CRC that plays a potential oncogenic role associated with worse outcome and resistance to oxaliplatin. Moreover, this alteration defines a subgroup of patients who could benefit from therapies containing PP2A activators such as FTY720.