This work examines the influence of copper doping on the structure, optoelectronic properties, and photocatalytic performance of transition metal double perovskite oxides of the Ba2Ca0.67Nb1.33O6 (BCN) family. A solid-state synthesis method was used to achieve partial substitution of Cu2+ in the Nb5+ site (Ba2Ca0.67Nb1.33-xCuxO6-δ, x = 0.05, 0.13, 0.26) and Ca2+ site (Ba2Ca0.67-xCuxNb1.33O6-δ, x = 0.13). The incorporation of copper in BCN resulted in significant changes in the structure and optoelectronic properties. Major differences were observed when Cu atoms occupied the Nb-site versus the Ca-site. The parent compound and Ca-site doped compounds crystallized in a P3 1 space group while Nb-site doped compounds showed increasing Fm3 mixed phase character. Cu doping resulted in a dramatic shift of the absorption band edge towards the visible region. Ca-site doping resulted in a primary band edge shift from 3.8 eV to 2.1. For the Nb-site doped compounds, the primary band-edge remained at 3.8 eV but a secondary band-edge appeared which progressively shifted to smaller energies as Cu doping increased. Ba2Ca0.67Nb1.20Cu0.13O6-δ and Ba2Ca0.67Nb1.07Cu0.26O6-δ exhibited strong absorption of near-infrared photons of wavelength well beyond 800 nm. The double perovskite oxides were utilized as photoanodes for photoelectrochemical water splitting, exhibiting both a visible photoresponse (until wavelengths as low as 520 nm) and good photochemical stability. This work showcases the possibility of forming new low band gap multi-element inorganic semiconductor oxides constituted entirely of base metals and oxygen.