We report the shape evolution process of Cu2O nanocrystals upon slow oxidation of Cu under ambient conditions, yielding novel hexagonal and triangular platelike morphologies. The shape of the obtained nanocrystals evolves from hexagonal to triangular to octahedral; the growth patterns are governed by kinetically and thermodynamically controlled growth. Preferential adsorption of I- on {111} planes of Cu2O nanoparticles induced the selective crystal growth of metastable platelike structures with {111} faces as the basal planes. On aging, the growth process appeared to shift into the thermodynamic regime and the thermodynamically stable octahedral shape is obtained. The possible growth mechanisms were investigated by varying the synthetic conditions. The band gap of Cu2O nanooctahedrons was determined by the classical Tauc approach to be 2.24 eV, which is blue shifted with respect to the bulk Cu2O value (2.17 eV). Results suggest that the slow oxidation process and use of crystallographic selective surfactants are essential for the appearance of anisotropic metastable shapes. In general, surface energy control by surfactant molecules might provide a convenient channel for tailoring nanocrystal shapes of metal oxides.