Endosymbionts are closely adapted to the biology of their insect hosts. In recent years, the routes and mechanisms involved in the horizontal transmission of insect endosymbionts has been an important topic in entomological research. A growing number of studies have revealed that insects with no close taxonomic affinity are infected with the same, or similar, endosymbionts. This indicates that horizontal transmission of insect endosymbionts is ubiquitous in nature, and that plant-mediated horizontal transmission of insect endosymbionts is a pathway for this. Recipient insects either of the same, or different, species can acquire endosymbionts from infected host plants; i.e. endosymbionts are transmitted from source insects to recipients via host plants. This paper reviews the mechanisms and processes involved in the plant-mediated horizontal transmission of insect endosymbionts, as well as the biological and ecological effects of endosymbionts on recipient hosts. We hope this review will provide a valuable reference for research on insect endosymbionts and pest control.