A semiconductor device having a decoupling capacitor and a method of fabricating the same are provided. The semiconductor device includes a semiconductor substrate having a cell region, a first peripheral circuit region, and a second peripheral circuit region. At least one channel trench is disposed in the cell region of the semiconductor substrate. At least one first capacitor trench is disposed in the first peripheral circuit region of the semiconductor substrate, and at least one second capacitor trench is disposed in the second peripheral circuit region of the semiconductor substrate. A gate electrode is disposed in the cell region of the semiconductor substrate and fills the channel trench. A first upper electrode is disposed in the first peripheral circuit region of the semiconductor substrate and fills at least the first capacitor trench. A second upper electrode is disposed in the second peripheral circuit region of the semiconductor substrate and fills at least the second capacitor trench. A gate dielectric layer is interposed between the channel trench and the gate electrode. A first dielectric layer is interposed between the semiconductor substrate of the first peripheral circuit region having the first capacitor trench and the first upper electrode and has the same thickness as the gate dielectric layer. A second dielectric layer is interposed between the semiconductor substrate of the second peripheral circuit region having the second capacitor trench and the second upper electrode and has a different thickness from the first dielectric layer.