We report on the detection of cysteine in water using an extended-gate-type organic field-effect transistor (OFET). The fabricated OFET device can be operated below 3 V. The portion of the designed device used for cysteine detection is the extended-gate electrode prepared by vacuum deposition of a gold (Au) layer. Cysteine can bind to the Au electrode surface through chemisorption. By this technique, we have successfully observed a shift in the threshold voltage of the OFET upon the addition of cysteine in an aqueous solution, whereby the detection limit for cysteine was found to be 39 ppb.