A semiconductor device includes a wiring substrate, a semiconductor element mounted on the wiring substrate, a heat dissipation plate arranged on an upper surface of the semiconductor element with an adhesive arranged in between, and an encapsulation resin filling a gap between the heat dissipation plate and the wiring substrate. The heat dissipation plate includes a body and a projection. The body is overlapped with the semiconductor element in a plan view and has a larger planar shape than the semiconductor element. The projection is formed integrally with the body. The projection projects outward from an end of the body and is located below the body. The encapsulation resin covers upper and lower surfaces of the projection. The body includes an upper surface exposed from the encapsulation resin.