This study aimed to examine the marginal seal between various commercial temporary restorative materials and exposed dentin/built-up composite. Sixty bovine incisors were cut above the cemento-enamel junction, and half of the dentin was removed to form a step, which was built up using flowable resin composite. The root canals were irrigated, filled with calcium hydroxide, and sealed using one of six temporary sealing materials (hydraulic temporary restorative material, temporary stopping material, zinc oxide eugenol cement, glass-ionomer cement, auto-cured resin-based temporary restorative material, and light-cured resin-based temporary restorative material) (n = 10 for each material). The samples were thermocycled 500 times and immersed in an aqueous solution of methylene blue. After 2 days, they were cut along the long axis of the tooth and the depth of dye penetration was measured at the dentin side and the built-up composite side. For the margins of the pre-endodontic resin composite build-up, the two resin-based temporary restorative materials showed excellent sealing. Hydraulic temporary restorative material had a moderate sealing effect, but the sealing effect of both zinc oxide eugenol cement and glass-ionomer cement was poorer.