Food waste is a global challenge that needs to be mitigated in the development of more sustainable societies. From manufacturers to customers, food biosensors could effectively reduce the amount of discarded food and provide more precise predictions of freshness with respect to pre-decided expiration dates. In this study, we developed a novel organic electrochemical transistor (OECT)-based xanthine biosensor. The OECT-based biosensor is based on the p-type conjugated polymer, p(g42T-TT) as the channel, and incorporated xanthine oxidase (XOD) as the biorecognition element. The OECT thus acts as a transducer and amplifier of the enzymatic oxidation of xanthine. Real-time monitoring of xanthine using the OECT-based biosensor led to a linear range between 5 and 98 μM (R2=0.989), 3.28 μM limit of detection, and high sensitivity up to 21.8 mA/mM. Real sample tests showed that the biosensor can detect the accumulation of xanthine in fish meat from 0 to 6 days of degradation. Interference tests with ascorbic acid and uric acid and spike-and-recovery tests with fish samples indicated that as-designed biosensors have good selectivity and accuracy. The developed biosensors show great potential for point-of-care testing applied to food monitoring.