Well‐defined Cu2O nanocubes with tunable dimensions and physicochemical properties have been prepared using a simple one‐pot reaction. Reduction of Cu(II) salts by ascorbic acid in the presence of PEG as a structure‐directing agent affords crystalline Cu2O nanocubes of between 50 to 500 nm. Optical band gap, band energies, charge‐carrier lifetimes and surface oxidation state systematically evolve with nanocube size, and correlate well with visible light photocatalytic activity for aqueous phase phenol degradation and H2 production which are both directly proportional to size (doubling between 50 and 500 nm). HPLC reveals fumaric acid as the primary organic product of phenol degradation, and selectivity increases with nanocube size at the expense of toxic catechol. Apparent quantum efficiencies reach 26 % for phenol photodegradation and 1.2 % for H2 production using 500 nm Cu2O cubes.