Perovskite-type compounds have been widely studied due to their multiple properties, such as high mechanical resistance, mixed ionic and electronic conductivity and photocatalytic activity. Specifically, an energy band gap (Eg) between 1.4 and 2 eV is sought for photocatalytic applications, such as clean hydrogen generation or solar energy conversion into electrical energy. Sr2FeNbO6 (SFNO) and SrNbO3 (SNO) perovskite compounds have great potential in these applications, exhibiting an Eg of 1.9 and 2.1 eV, respectively. SFNO is a semiconductor, while SNO is an unusual material with photocatalytic properties originating from its optical energy gap and metal behavior. However, the determination of Eg in SNO is controversial. In this work, a systematic study within the DFT framework using the LDA and Hubbard (U) correction has been presented to reveal the origin of Eg in SNO through substitutions in the Sr2Fe1–xNb1+xO6 (SFNxO) system with x = 0.00, 0.25, 0.50, 0.75 and 1.00. The electronic and magnetic properties and crystal structure were studied. A shift to lower energies in the valence and conduction bands in SFNO has been identified upon increasing the Nb content, showing the inter-band origin of Eg in SNO. The Eg value is reduced up to ∼1.5 eV for x = 0.25, 0.50 and 0.75 with trap states in the Eg. In this system the magnetic behavior has been predicted to be antiferromagnetic (AFM) or ferrimagnetic. However, for x = 0.50, it presents a degenerate state with AFM and ferromagnetic behavior. The magnetic moment of Nb in this system can be modulated as a function of the Fe content, which broadens the technological applications. Finally, our crystal structure study has indicated an increase in the volume due to the oxidation states of Fe and Nb, and the orthorhombic structure in the system is not changed, indicating that the solid solution synthesis of these materials can be successfully carried out.