Gate-all-around integrated circuit structures having an insulator substrate, and methods of fabricating gate-all-around integrated circuit structures having an insulator substrate, are described. For example, an integrated circuit structure includes a semiconductor fin on an insulator substrate. A vertical arrangement of horizontal nanowires is over the semiconductor fin. A gate stack surrounds a channel region of the vertical arrangement of horizontal nanowires, and the gate stack is overlying a channel region of the semiconductor fin. A pair of epitaxial source or drain structures is at first and second ends of the vertical arrangement of horizontal nanowires and the semiconductor fin.