Fiber organic electrochemical transistors (OECTs) have received extensive attention in wearable and implantable biosensors because of their high flexibility and low working voltage. However, the transconductance of fiber OECTs is much lower compared with the planar counterparts, leading to low sensitivity. Here, we developed fiber OECTs in a coaxial configuration with microscale channel length to achieve the highest transconductance of 135 mS, which is one to two orders of magnitude higher than that of the state-of-the-art fiber OECTs. Coaxial fiber OECT based sensors showed high sensitivities of 12.78, 20.53 and 3.78 mA/decade to ascorbic acid, hydrogen peroxide and glucose, respectively. These fiber OECTs were woven into a fabric to monitor the glucose in sweat during exercise and implanted in mouse brain to detect ascorbic acid. This coaxial architectural design offers an effective way to promote the performance of fiber OECTs and realize highly sensitive detection of biochemicals.