Organic electrochemical transistors (OECTs) rely on volumetric ion‐modulation of the electronic current to provide low‐voltage operation, large signal amplification, enhanced sensing capabilities, and seamless integration with biology. The majority of current OECT technologies require multistep photolithographic microfabrication methods on glass or plastic substrates, which do not provide an ideal path toward ultralow cost ubiquitous and sustainable electronics and bioelectronics. At the same time, the development of advanced bioelectronic circuits combining bio‐detection, amplification, and local processing functionalities urgently demand for OECT technology platforms with a monolithic integration of high‐performance iontronic circuits and sensors. Here, fully printed mask‐less OECTs fabricated on thin‐film biodegradable and compostable substrates are proposed. The dispensing and capillary printing methods are used for depositing both high‐ and low‐viscosity OECT materials. Fully printed OECT unipolar inverter circuits with a gain normalized to the supply voltage as high as 136.6 V⁻¹, and current‐driven sensors for ion detection and real‐time monitoring with a sensitivity of up to 506 mV dec⁻¹, are integrated on biodegradable and compostable substrates. These universal building blocks with the top‐performance ever reported demonstrate the effectiveness of the proposed approach and can open opportunities for next‐generation high‐performance sustainable bioelectronics.