We report the synthesis of a phosphorus grafted chitosan functionalized graphene oxide (GO) based nanocomposite (PCG) as a highly potent flame retardant (FR). A PCG-coated cloth (PCGC) sample on exposure to flame retained its original size and shape; moreover, it burned to release only a trivial quantity of smoke and did not catch fire for 23 min, whereas the cloth sample coated with chitosan and graphene oxide [chitosan coated cloth (CC) and graphene oxide coated cloth (GC)] burned completely within 20 s. Uncoated cotton cloth without any coating burned entirely within 5 s, leaving a slight amount of black residue. The flame-retardant (FR) efficacy of the PCG-coated cloth sample (PCGC) was further analyzed using detailed flame criteria, like the limiting oxygen index test (LOI), vertical flammability test (VFT), and stability at elevated temperatures (e.g., ∼1500 °C). According to the flammability evaluation, the PCGC passed the vertical flammability test (VFT) with a residual char length of 1 cm, increasing the limiting oxygen index value from 17.6% for uncoated cloth to 51.2%. The PCGC's tensile strength increased from 27.52 MPa to 42.15 MPa, and its elongation at break improved from 25.7% to 45.4%. Even after five washing cycles, the PCGC showed flame retardancy. In addition to cotton cloth, wood was employed to test the flame retardancy of the synthesized nanocomposite, and favourable results were observed. This work reports the first-ever attempt to formulate a phosphorus-modified GO chitosan nanocomposite (PCG) utilizing a simple, low-cost synthetic path vis-à-vis the prior work, and it has a potential application as a flame-retardant material for fire protection.