A semiconductor device including a relatively thin interposer excluding a through silicon hole and a manufacturing method thereof are provided. The method includes forming an interposer on a dummy substrate. The forming of the interposer includes, forming a dielectric layer on the dummy substrate, forming a pattern and a via on the dielectric layer, and forming a seed layer at the pattern and the via of the dielectric layer and forming a redistribution layer and a conductive via on the seed layer. A semiconductor die is connected with the conductive via facing an upper portion of the interposer, and the semiconductor die is encapsulated with an encapsulant. The dummy substrate is removed from the interposer. A bump is connected with the conductive via facing a lower portion of the interposer.