The C–O bond dissociation of the CO2 molecule via the reverse water gas shift reaction is crucial for several reactions used as renewable alternatives for fuel synthesis. However, our atomistic understanding of this process on transition metal (TM) clusters, where quantum-size effects might play a significant role, is far from complete. Here, we addressed the C–O bond dissociation by redox and carboxyl routes on 13-atom TM (Fe, Co, Ni, Cu) clusters using density functional theory calculations and the climbing image nudged elastic band algorithm. From the potential energy profiles, we found that CO2 is prone to dissociate into adsorbed CO via the redox route with lower activation energies, Ea’s, than the carboxyl route on all studied TM13 systems. Our results suggested that the smaller activation barrier found on the Co13 cluster is due to the stronger adsorption exhibited for both CO2 and O. By increasing the d-state occupation (from Fe to Cu), the Ea differences between CO2 dissociation and COOH formation decrease. We associated this behavior with a decrease in the (CO2 + H) adsorption energy from Fe13 to Cu13 that facilitates the COO–H bond formation and H–TM bond cleavage, i.e., favoring the carboxyl route. Also, our analyses indicate that the adsorption energies of the CO2 and trans-COOH species are the best descriptors for the C–O bond dissociation via the redox and carboxyl routes, respectively.