Here we report the synthesis of graphitic carbon nitride (g-C3N4)/brownmillerite KBiFe2O5 (KBFO) based heterojunction photocatalyst. The heterojunction composite revealed rod shaped KBFO embedded on the surface of g-C3N4. The distinct phases of individual compounds are revealed through X-ray diffraction. An effective reduction in bandgap energy of KBFO/g-C3N4 (KCN) composite in comparison to pristine g-C3N4 confirmed the formation of heterojunction. Photocatalytic performance of as developed heterostructures is tested and compared by degrading common organic effluent methylene blue (MB). 25 wt% of g-C3N4 content in KBFO sample shows a remarkable improvement (92% degradation efficiency) over pure KBFO (77%) and g-C3N4 (54%). The electrode composed of 25 wt% g-C3N4 content in KBFO also reveals significant photoelectrochemical response (0.3 mA/cm2) over KBFO (1.5 μA/cm2). The improvement was attributed to an effective electron-hole separation in the heterojunction. These stable and reusable KBFO/g–C3N4 heterostructures hold promising future materials for energy and environmental applications. © 2022 Elsevier B.V.