Cell-to-cell communication is fundamental to multicellular organisms, as well as to unicellular organisms living in a microbiome. It is thought to have evolved as a stress- or quorum-sensing mechanism in unicellular organisms. A unique cell-to-cell communication mechanism that uses reactive oxygen species (ROS) as a signal (termed the ROS wave) was recently identified in flowering plants. This process is essential for systemic signaling and plant acclimation to stress and can spread from a small group of cells to the entire plant within minutes. Whether this signaling process is found in other organisms is however unknown. Here we report that a ROS-mediated cell-to-cell signaling process, like the ROS wave, can be found in ferns, mosses, multicellular and unicellular algae, amoeba, and mammalian cells. We further show that this process can be triggered by extracellular H2O2 application and blocked by inhibition of NADPH oxidases, and that at least in unicellular algae growing as a lawn on an agar plate, it communicates important stress-response signals between cells. Taken together, our findings suggest that an active process of cell-to-cell ROS signaling, like the ROS wave, evolved before unicellular and multicellular organisms diverged. This mechanism could have communicated an environmental stress signal between cells and coordinated the acclimation response of cells living in a community. The finding of a signaling process, like the ROS wave, in mammalian cells further contributes to our understanding of different diseases and could impact the development of new drugs that target, for example cancer or heart disease.