Patch-type hydrogel electrodes have received increasing attention in biomedical applications due to their high biocompatibility and conformal adherence. However, their poor mechanical properties and non-uniform electrical performance in a large area of the hydrogel electrode should be improved for use in wearable devices for biosignal monitoring. Here, we developed self-adherent, biocompatible hydrogel electrodes composed of biodegradable gelatin and conductive polymers for electrocardiography (ECG) measurement. After incorporating conductive poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) into gelatin hydrogels crosslinked by natural crosslinkers (genipin), the mechanical properties and electrical conductivity of the hydrogel electrodes were improved and additionally optimized by adjusting the amounts of crosslinker and PEDOT:PSS, respectively. Furthermore, the effect of dimethyl sulfoxide, as a dopant, on the conductivity of hydrogels was investigated. The gelatin-based, conductive hydrogel patch displayed self-adherence to human skin with an adhesive strength of 0.85 N and achieved conformal contact with less skin irritation compared to conventional electrodes with a chemical adhesive layer. Eyelet-type hydrogel electrodes, which were compatible with conventional ECG measurement instruments, exhibited a comparable performance in 12-lead human ECG measurement with commercial ECG clinical electrodes (3M Red Dot). These self-adherent, biocompatible, gelatin-based hydrogel electrodes could be used for monitoring various biosignals, such as in electromyography and electroencephalography.