Currently, more and more evidence is accumulating in favor of plant hormesis (i.e., low-dose stimulation and high-dose inhibition) when exposed to various nanomaterials (NMs) (metal NMs, carbon-based NMs, polymeric NMs, and hybrid NMs). However, the mechanisms underlying the stimulating and inhibitory effects of NMs have not been sufficiently studied. Here, the regularities of plant hormesis, including its mechanisms, are discussed in terms of the concept of environmental hormesis. The qualitative features of nanomaterial effects on plants and their relationship with direct and indirect hormetic stimulation of plants by NMs in low doses are considered. It has been shown that along with direct stimulation, any nanomaterials activate non-specific and specific defense mechanisms of plants, which largely determine their hormetic effects. The concept of environmental hormesis can be useful for understanding the patterns and mechanisms underlying the effects of NMs on plants, as well as for improving their efficiency and environmental safety.