Right-sided metastatic colorectal cancer (mCRC) patients have poor prognosis and achieve limited benefit from first-line doublets plus a targeted agent. In this unplanned analysis of the TRIBE study, we investigated the prognostic and predictive impact of primary tumor sidedness in mCRC patients and the differential impact of the intensification of the chemotherapy in subgroups defined according to both primary tumor sidedness and RAS and BRAF mutational status. Patients were randomized to receive upfront 5-fluoruracil, leucovorin, and irinotecan (FOLFIRI) plus bevacizumab or 5-fluoruracil, leucovorin, oxaliplatin, and irinotecan (FOLFOXIRI) plus bevacizumab. Tumors were defined as right- or left-sided if they originated from the caecum to the transverse colon or within the splenic flexure and beyond, respectively. Patients with available information about both primary sidedness and RAS and BRAF status were included in the present analysis. Progression-free survival (PFS), overall survival (OS) and RECIST response rate were assessed according to tumor location and RAS and BRAF mutational status. Information about primary sidedness and RAS and BRAF status was available for 358 (70.5%) out of 508 randomized patients. Patients with right-sided tumors (N=173) presented shorter OS 23.7 versus 31.0months, HR=1.42 (95% CI 1.09–1.84), P=0.010 and a trend toward shorter PFS 10.2 versus 11.5months, HR=1.24 (95% CI: 0.98–1.56), P=0.083 than those with left-sided tumors (N=185), but these associations were no longer evident when adjusting for RAS and BRAF status. Patients with right-sided tumors achieved more relative benefit from the intensification of the chemotherapy backbone in terms of both PFS (HR=0.59 versus 0.89, P for interaction=0.099) and OS (HR=0.56 versus 0.99, P for interaction=0.030) and this advantage was independent of their RAS and BRAF status. FOLFOXIRI plus bevacizumab may be regarded as a preferred first-line treatment option for clinically selected patients with right-sided metastatic colorectal cancer irrespective of their RAS and BRAF mutational status. Trial registration: clinicaltrials.gov identifier NCT00719797.