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Non‐invasive electrodes for recording and delivering electric signals to the human body are crucial for health monitoring and rehabilitation applications. However, high‐fidelity signal recording or delivery with epidermal electrodes remains a challenge due to the need for shape customization, to account for the variance of body morphology among individuals, and the need for conformal contact, to accommodate creviced skin surfaces, intricate curves, and moving bodies. In this study, a conductive and self‐adhesive hydrogel for direct ink writing of wearable electrodes on the skin is presented, utilizing physical cross‐linking mechanisms between bio‐based polymers. With a fast gelation time and a facile fabrication method, the printed hydrogel achieves a 0.40 mm resolution via handheld 3D printers. Compared with silver/silver chloride (Ag/AgCl) coated gel electrode standards, the hydrogel electrode formed in situ achieves a higher signal‐to‐noise ratio by 88%, for the monitoring of forearm muscle biopotential and decreases the required current from 3.5 to 2.25 mA, for the functional electrical stimulation for eye closure. The lowered contact impedance of the hydrogel electrode is attributed to its sol–gel transition in situ on the skin, demonstrating its potential to enable future healthcare applications with improved personalization, efficiency, and comfort. [ABSTRACT FROM AUTHOR]