In situ monitoring of biomedical parameters with implantable sensors can provide information to trigger interventional or therapeutic actions. However, these sensors require bulky components for power or for interrogation that hinder miniaturization. We have proposed a wireless sensing method based in passive rectification of high frequency current bursts that flow through the tissues by volume conduction. Here we report the evaluation of a 0.98 mm-thick, passive, and flexible temperature sensor based on this method. The injectable microsensor obtained an accuracy of ±2.1%. This opens the possibility of continuous and in situ temperature sensing with minimal invasiveness.