The present invention is an adhesive composition including a Polyisobutylene-based resin (A), an energy ray-curable resin (B), and a tackifier (C), the energy ray-curable resin (B) including a difunctional acrylate that includes a divalent alicyclic hydrocarbon group, the adhesive composition including the energy ray-curable resin (B) in a ratio of 5 to 59 parts by mass based on 100 parts by mass of the Polyisobutylene-based resin (A), and including the tackifier (C) in a ratio of 0.1 to 40 parts by mass based on 100 parts by mass of the Polyisobutylene-based resin (A); and an adhesive sheet including a release sheet, and an adhesive layer that is formed on the release sheet, the adhesive layer being formed using the adhesive composition; and an electronic device including a transparent substrate, a device, and a sealing material, the sealing material being formed using the adhesive composition.