Abstract Data processing through artificial synapses is gaining attention owing to the emergence of neuromorphic computing. Analog processing via these synapses can simultaneously handle large volumes of data; however, it is susceptible to interference from various environmental factors. Specifically, temperature changes can significantly affect overall signal characteristics, leading to substantial errors. Herein, an organic heterojunction‐based artificial synapse is presented that is capable of light–temperature antagonistic operations. The properly aligned band structure and trap sites, which are facilitated by oxygen penetration, enable the implementation of controlled synaptic characteristics, depending on temperature and light conditions. An increase in temperature resulted in a thermally enhanced synaptic current, while light irradiation reduced the synaptic current, with the reduction degree being dependent on the light intensity. Finally, a biomimetic analog processor system capable of signal stabilization under drastic temperature changes is implemented. The artificial synapse, which operates using a light–temperature antagonistic operation, can significantly expand the potential applications of artificial intelligence hardware.