One of the puzzles in biology is how mammalian sperm cells can sense temperature changes as small as <0.0006°C and respond to them by thermotaxis (1, 2). This question is general and highly intriguing because, on the basis of the current knowledge, it is impossible to envisage how a cell can sense such tiny temperature differences by the known thermosensors in mammals — ion channels. Another puzzle in biology is the presence of opsins, known to act as photosensors in the eye, in non-photosensitive organs such as kidneys and lungs. We show that these two questions are likely linked, resolving the first question and proposing a solution to the second one. Thus, we demonstrate that the sperm temperature sensitivity is achieved by a family of thermosensors that involves the visual opsins, belonging to the G-protein coupled receptor (GPCR) superfamily. We identify the visual opsins in human sperm cells, we provide evidence for their involvement in human sperm thermotaxis, and we reveal the signaling pathway triggered by them. Using rhodopsin-knockout mice, we also show the involvement of rhodopsin in mouse sperm thermotaxis. The results further suggest that opsins can act as both photosensors and thermosensors, depending on their location, and that the hitherto unexplained expression of opsins in non-photosensitive organs is for temperature sensing.