Climate measurements are needed at a scale at which organisms live and die. Currently available climate sensors, however, are not well suited for long-term field measurements at such a scale. We have therefore developed a new temperature and moisture logger, the Temperature-Moisture-Sensor (TMS), which we designed for a wide range of ecological applications. The device mimics a small herbaceous plant. Its belowground part houses a patented, proprietary soil moisture sensor working on the time-domain transmission principle. Air, surface and soil temperatures are measured simultaneously by three independent sensors. The TMS data logger has a large memory and long battery life, so it is suitable for taking long-term microclimate measurements in the field. With a data acquisition interval of 15 min, it has sufficient memory to last for almost 15 years. We have thoroughly tested the TMS logger both in the laboratory and in demanding field conditions ranging from tropical rain forests of Africa to high-elevation cold deserts of the Himalayas. The device has provided microclimate measurements in a wide range of environmental conditions and has also performed well in controlled laboratory settings. The key added value of the TMS logger is that it concurrently measures soil moisture as well as soil, surface and air temperature at a biologically relevant scale. It is also able to continuously measure the microclimate for several years even in the most extreme conditions. The device can therefore be used to build extensive tailored field measurement networks providing crucial data about microclimate conditions shaping biological processes in the face of climate change.