The present disclosure relates to a dielectrically isolated semiconductor device and a method for manufacturing the same. The dielectrically isolated semiconductor device includes a semiconductor substrate, a first semiconductor layer above the semiconductor substrate, a second semiconductor layer above the first semiconductor layer, a semiconductor island in the second semiconductor layer, and a first dielectric isolation layer surrounding a bottom and sidewalls of the semiconductor island. The first dielectric isolation layer includes a first portion which is formed from a portion of the first semiconductor layer and extending along the bottom of the semiconductor island, and a second portion which is formed from a portion of the second semiconductor layer and extending along the sidewalls of the semiconductor island. The dielectrically isolated semiconductor devices needs no an SOI wafer and reduces manufacturing cost.