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In this paper, a simple thermal decomposition synthesis route for a carbon–nitrogen–sulfur (C–N–S) co-doped titanium dioxide (TiO2) mischcrystal with an isobandgap property for visible light applications using urea as the C, and N source and titanium(IV) sulfate [Ti(SO4)2] as S source is reported. The C–N–S co-doped TiO2 mischcrystal exhibited a unique isobandgap characteristic and prominent red-shift in the ultraviolet-visible absorption spectrum and a much narrower band gap (2.42 eV and 1.39 eV) because of defects and impurity levels which were confirmed using powder X-ray diffraction, diffuse reflectance spectroscopy (DRS), photoluminescence, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The excellent photocatalytic performance of the C–N–S co-doped TiO2 mischcrystal samples was demonstrated using degradation of methylene blue and phenol under visible light. The apparent degradation rate of black TiO2 is 4–7 times higher than that of C–N–S co-doped TiO2 (anatase or rutile) under visible light irradiation. The valence band XPS and DRS results were carefully examined to understand the band gap reduction of C–N–S co-doped TiO2. This study confirms that the band gap structure of C–N–S co-doped TiO2 is determined by the mixed crystalline structure and the formation of impurity levels and defective levels. A new idea is presented that will be useful for the design of band gap structure for photocatalysts with higher photocatalytic activities under visible light irradiation.