Electrochemical polymerization is a versatile method for rapid deposition of conducting polymer (CP) films. The target substrates have, however, been limited to planar, metallic surfaces; hence, the devices that integrate electropolymerized CP films have predominantly been passive electrodes. In this work, it is shown that electrochemical polymerization has a high degree of freedom, which allows growing biofunctionalized CP films in microscale transistor channels. CP films are electrochemically deposited from two monomers, namely, 3,4‐ethylenedioxythiophene (EDOT) and hydroxymethyl EDOT (EDOTOH), inside the channel of an organic electrochemical transistor (OECT). In aqueous electrolytes, the copolymer p(EDOT‐ran‐EDOTOH) shows excellent charging capability and OECT performance. The presence of hydroxyl groups facilitates stable incorporation of catalytic enzymes in the copolymer matrix during electropolymerization, rendering OECT channels biologically functionalized. The transistor channels made of CP films with the entrapped enzyme show output characteristics that change with respect to the concentration of its target metabolite. In the form of a miniaturized, single chip, the multi‐transistor platform simultaneously measures glucose, cholesterol, and lactate concentrations of a given fluid. With the ability to grow and pattern CPs functionalized with biorecognition units on miniaturized areas, this technique promises for the development of multiplexed platforms for electronic biosensing.