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