Apexification is a method used to induce a calcified barrier in a root with an open apex or to continue the apical development of an incompletely formed root in teeth with necrotic pulp. In this case report, a necrotic mandibular second molar with periapical involvement and an open apex was treated in a single visit with one-step apexification. Standard root canal irrigation was performed, and instead of Gutta-Percha (GP) obturation, Mineral Trioxide Aggregate (MTA) was filled in the apical third, followed by gutta-percha in the middle and coronal thirds. Post-operative radiographic examination showed complete sealing of the apex with MTA.