The energy crisis has worsened with the increase in population; to overcome this crisis, self-powered devices are required urgently. Human beings have also become increasingly health conscious and require modern equipment to monitor their body conditions. Body temperature is a measure of the thermoregulation in the body and also indicates other health conditions. For continuous body temperature monitoring, smart sensors are the best choice. In this work, we fabricate an ultrathin (150 nm) and flexible temperature sensor using thermoelectric tellurium nanoparticles (Te-NPs) sputtered on a thin Al substrate that can detect the exact surface temperature relative to its ambient temperature measurement. The sensor can sense both hot and cold surfaces and shows a linear response with increasing or decreasing temperature measurement. Because of its compact size and flexibility, it can conform to any surface for temperature. For a temperature gradient of 75 ℃, the sensor gives an output voltage of 0.4 mV and can even accurately measure body temperature with only a human finger touch. Such a flexible and thin device can be used in the day-to-day monitoring of the human body or any other surface whose temperature should be determined before touching it.