We present here the results of a paleoseismic investigation carried across a ~10 m high fault scarp at Panijhora village, West Bengal in northeastern India. Accelerator Mass Spectrometer analyzed 14C radiocarbon age constraints from six detrital charcoal samples ranging between 1688 B.C. and A.D. 1152 are consistent with the great medieval earthquake of A.D. 1255 that is interpreted to have produced a minimum observed fault slip of ~5 m in the trench exposure. Recalibration of radiocarbon ages from previous studies at Harmutty, Nameri, and Marha in the eastern Himalaya using Bayesian statistical analyses further substantiates the possibility that the A.D. 1255 earthquake might have ruptured the Himalayan front over a length of ~800 km from ~85.87° to 93.76°E longitudes. Key Points We conclude that the 1255 medieval earthquake was a giant event with a 800 km rupture length We validate that the Himalayan continental subduction zone is capable of producing giant earthquakes approaching Mw 9.0 Speculation of a major event in the eastern Himalaya