Self-aligned gate endcap (SAGE) architectures with vertical sidewalls, and methods of fabricating self-aligned gate endcap (SAGE) architectures with vertical sidewalls, are described. In an example, an integrated circuit structure includes a semiconductor fin having sidewalls along a length of the semiconductor fin, each sidewall tapering outwardly from a top of the semiconductor fin toward a bottom of the semiconductor fin. A gate endcap isolation structure is spaced apart from the semiconductor fin and has a length parallel with the length of the semiconductor fin. The gate endcap isolation structure has a substantially vertical sidewall laterally facing one of the outwardly tapering sidewalls of the semiconductor fin.