Hyperaccumulators are plants that accumulate toxic elements to extraordinary levels. Selenium (Se) hyperaccumulators can contain 0.1–1.5% of their dry weight as Se, levels toxic to most other organisms. In this review we summarise what is known about the ecological functions and implications of Se (hyper)accumulation by plants. Selenium promotes hyperaccumulator growth and also offers a plant several ecological advantages through negative effects on Se-sensitive partners. High tissue Se levels reduce herbivory and pathogen infection, and high-Se litter deposition can inhibit neighbouring plants. There is no evidence for a cost of hyperaccumulation in terms of reproductive functions or pollinator visitation. Hyperaccumulators offer a niche for Se-tolerant herbivores, pollinators, microbes and neighbouring plants. They may even facilitate these partners through Se enrichment: neighbouring plants with elevated Se levels enjoy enhanced growth and reduced herbivory. Through combined negative and positive effects on ecological partners, Se hyperaccumulators likely affect local plant, microbial and animal species composition and richness, favouring Se-tolerant species at different trophic levels. By locally concentrating Se and altering its chemical form, Se hyperaccumulators likely play an important role in Se entry into, and Se cycling through, seleniferous ecosystems. These findings are of significance since they provide insight into the ecological reverberations of Se hyperaccumulation, and shed light on the possible selection pressures that have led to the evolution of this fascinating phenomenon. Better ecological insight will also help in the management of seleniferous areas and the agricultural production of Se-rich crops for phytoremediation or biofortification.