Using spectroscopic data taken with OSIRIS spectrograph at the 10.4m-GTC telescope, we discover that the orbital period of the secondary star of the black hole binary XTE J1118+480, with an orbital period of P_{orb}˜ 4.1 hr, is rapidly decreasing. We have derived an orbital period time derivative of dot P=-1.83±0.66 ms yr^{-1} in this system. Conventional models including magnetic braking and mass loss can marginally explained these observations, but require high magnetic fields in the secondary star and significant mass loss from the system. This result has relevant implications on the strength of magnetic braking in these binary systems and also put tight constraints on the evaporation of black holes in the context of Braneworld gravity theory.