Living functions depend on temperature: an optimal one may increase the rate of development and reproduction, while extreme ones may stop basic biochemical reactions affecting an organism irreversibly. Therefore, sensing temperature and thermoregulatory behaviours may have been core functions of the nervous system since its origin in multicellular animals. It is possible to shed light on this hypothesis by studying cnidarians. Cnidarians (e.g., jellyfish) and bilaterians split from a common ancestor, the first carrier of neurons, about 750 million years ago. Neural thermal responses and thermoregulatory behaviour are displayed in all bilaterian species so far studied in laboratory settings: from Protostomes (e.g., annelid, fly, nematode) to Deuterostomes (e.g., fishes, rodents, human and non-human primates). However, whether cnidarians display neural thermal responses and thermoregulatory behaviour are currently open questions. Here we sought an answer with the cnidarian Hydra vulgaris. In neurons expressing GCaMP6s, we found different types of calcium responses upon heating and re-cooling across the animal body, with the oral and aboral regions of the animals showing opposite response patterns. Moreover, we described an essay which enabled us to uncover a thermoregulatory behavioural response in this ancestral animal. Thus, we provide evidence in favour of an evolutionary early origin of neural thermal responses and thermoregulatory behaviour in nervous system carriers.