A synthesis of novel sulfur and phosphorous co-doped graphitic carbon nitride (SP-g-C3N4) covalently grafted polyaniline (PANI) metal-free nanocomposite via an in situ oxidative polymerization is reported here. The nanocomposite was characterized by various instrumentation techniques such as UV-DRS, FT-IR, XRD, SEM, HR-TEM, EDX, and SEM mapping analysis. The as-prepared SP-g-C3N4-PANI metal-free nanocomposite shows an outstanding photocatalytic activity for the dedyeing of methylene blue (MB) under visible light irradiation. The nanocomposite catalyst greatly promotes the charge separation efficiency of photogenerated electrons and holes compared to its single counterpart, viz., PANI, S-g-C3N4, P-g-C3N4, and SP-g-C3N4, for catalytic degradation. The present novel approach provides a new insight into the design and development of nanocomposite materials for the removal of toxic contaminants for public safety and security. Also, study of this kind will undoubtedly expedite new research on solar-driven water splitting.