One Health is one of the greatest concepts of this century. By acting on human health, for example, with indiscriminate use of antibiotics, humans can affect the environment and animal health. Moreover, environmental pollution involves global impacts on animal and human health. However, with their nanosize and their huge specific area, nanomaterials can act on both humans and the environment. First, nanoparticles and nanomaterials are powerful tools for medical device sensors, as well as radiosensitizers or even in theranostic applications. The biological mechanism being generally highly specific and sensitive, biosensing can be enhanced by the large surface area of nanoparticles. Moreover, theranostic being the meeting between medical sensing and personal healthcare, the environmental impact of human health can be reduced drastically. That is why in recent decades, theranostic became the privileged way of treating patients and nanomaterials have taken their full place. Second, nanomaterials can remove pollutants from wastewater; for example, organic dyes, which are a big problem worldwide, especially from textile industries. These azo or polyaromatic dyes can be degraded by nanoparticles, mostly thanks to hydroxyl radicals emission. The degradation problem could reside in the fact that the authors may not know all the degradation products and gases emitted, which could be toxic by themselves. The other way of dyes environmental remediation is their adsorption in nanomaterials, especially if these adsorbents can be removed magnetically from wastewater. Some problematic dyes or model dyes can be thus removed successfully from polluted water. With this in mind, and thanks to a better risk assessment of nanoparticles and nanomaterials, there is no doubt that nanosized devices should become a major theme for One Health development.