The synthesis of a new tetrathiafulvalene derivative with an electron-withdrawing benzothiadiazole moiety and its use in thin-film organic field-effect transistors (OFETs) are reported. Compared to reported OFETs with other TTF derivatives, a high hole mobility up to 0.73 cm2 V−1 s−1, low off-current and high on/off ratio up to 105 are demonstrated. In addition, the developed OFETs show fast responsiveness toward chemical vapors of DECP (diethyl chlorophosphate) or POCl3 which are simulants of phosphate-based nerve agents. In contrast to previously reported OFET-based sensors, off-current is used as the output signal, which increases quickly upon exposure to either DECP or POCl3 vapors. High sensitivity is demonstrated toward DECP and POCl3 vapors, with concentrations as low as 10 ppb being detected. These OFETs are also responsive to TNT vapor. The sensing mechanisms for the new type of OFET are discussed.