We study the ultrafast electronic relaxation of the proton transfer compound 2-(2′-hydroxyphenyl)benzothiazole (HBT) in a joint approach of femtosecond pump-probe experiments and dynamics simulations. The measurements show a lifetime of 2.6 ps for the isolated molecule in the gas phase in contrast to ∼100 ps for cyclohexane solution. This unexpected decrease by a factor of 40 for the gas phase is explained by ultrafast internal conversion to the ground state promoted by an inter-ring torsional mode. The quantum chemical calculations based on multireference configuration interaction clearly demonstrate that a S0/S1 conical intersection at a 90° twisted structure exists and is responsible for the ultrafast decay. The reaction path leading from the keto form of HBT to this intersection is practically barrierless on the S1 surface. The on-the-fly dynamics simulations using time-dependent density functional theory show that after electronic excitation to the S1 state and after fast excited-state proton transfer (30-50 fs), HBT reaches the region of the S1/S0 crossing within about 500 fs, which will lead to the observed 2.6 ps deactivation to the ground state. After the internal conversion, HBT branches in two populations, one that rapidly closes the proton transfer cycle and another (trans-keto) that takes ∼100 ps for that step.