<b><i>Objectives:</i></b> This phase II trial was aimed at assessing the safety and activity of capecitabine, oxaliplatin, and irinotecan (COI regimen) as a preoperative treatment for resectable gastric cancer (GC) or gastroesophageal junction (GEJ) cancer. <b><i>Methods:</i></b> Patients affected by T3-T4/N0-N+/M0 GC/GEJ cancer were treated with the COI regimen for 4 cycles followed by restaging and gastroresection with D2 lymphadenectomy. Four postoperative cycles were scheduled. The primary endpoint was pathological response rate according to Becker et al. [Cancer 2003;98:1521-1530]. The potential role of fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) as a predictive biomarker of pathological tumor response was assessed in a subgroup of 19 evaluable patients. <b><i>Results:</i></b> Between January 2011 and October 2015, a total of 40 patients were enrolled. After the preoperative phase, 36 out of 40 patients (90%) were considered eligible for surgery: 12 patients (30%) achieved a pathological response. The most frequent grade 3/4 adverse events were diarrhea (27%), nausea (25%), and fatigue (17%). Grade 3 neutropenia occurred in 7.5% of patients. A lower standard uptake value at baseline FDG-PET/CT was associated with pathological response. <b><i>Conclusion:</i></b> COI combination is active with a manageable toxicity profile in patients with resectable GC or GEJ cancer. FDG-PET/CT imaging as a surrogate biomarker of pathological response in this setting appears fascinating but should be further investigated.