Inosculation is a natural phenomenon in which trunks, branches or roots of two trees grow together in a manner biologically similar to the artificial process of grafting. The term is derived from the Latin roots in + ōsculārī, "to kiss into/inward/against" or etymologically and more illustratively "to make a small mouth inward/into/against"; trees having undergone the process are referred to in forestry as gemels, from the Latin word meaning "a pair".

It is most common for branches of two trees of the same species to grow together, though inosculation may be noted across related species. The branches first grow separately in proximity to each other until they touch. At this point, the bark on the touching surfaces is gradually abraded away as the trees move in the wind. Once the cambium of two trees touches, they sometimes self-graft and grow together as they expand in diameter. Inosculation customarily results when tree limbs are braided or pleached.

The term inosculation is also used in the context of plastic surgery, as one of the three mechanisms by which skin grafts take at the host site. Blood vessels from the recipient site are believed to connect with those of the graft in order to restore vascularity.