Wearable hydrogel-based electrocardiogram (ECG) sensors that continuously monitor cardiac electrical activity offer a promising alternative to traditional Ag/AgCl electrodes. However, their performance is compromised by thermal fluctuations that cause resistivity instability in the sensors, and therefore, unreliable readings. This project addresses this critical challenge by developing a computational model to simulate how temperature variations affect hydrogel resistivity and signal fidelity. Using COMSOL Multiphysics, we analyzed a multi-layered sensor system (epidermis, PDMS adhesive, Ecoflex coating, and hydrogels) under realistic conditions: skin temperature of 31°C and ECG voltages of 0.1 mV – 0.5 mV. The model coupled heat transfer and electric current physics to quantify resistivity changes and voltage attenuation. Our results revealed that hydrogel resistance decreases linearly with temperature due to enhanced ion mobility, confirming the relationship wherein sensor resistivity is a function of temperature with its thermal coefficient of resistance (TCR) being the dominant factor. Signal amplitude at the hydrogel-skin interface dropped by four orders of magnitudes, primarily due to impedance mismatches rather than material resistivity. Sensitivity analysis confirmed that thermal conductivity and heat capacity had negligible effects compared to the TCR. These findings underscore the need to optimize hydrogel composition – particularly polypyrrole (PPy) content – to mitigate thermal drift. To improve real-world performance, we recommend integrating thermal insulation into sensor casings and exploring alternative body placements (e.g. chest or ankles) with less temperature variability. This study provides a foundational framework for designing thermally stable hydrogel sensors, advancing their reliability for long-term cardiac monitoring and early diseases detection.