| Thézillieu | |
| Image Coat Of Arms: | Blason ville fr Thezillieu (Ain).svg |
| Arrondissement: | Belley |
| Canton: | Plateau d'Hauteville |
| Commune: | Plateau d'Hauteville |
| Insee: | 01417 |
| Postal Code: | 01110 |
| Coordinates: | 45.8928°N 5.6008°W |
| Elevation M: | 860 |
| Elevation Min M: | 706 |
| Elevation Max M: | 1123 |
| Area Km2: | 26.25 |
| Population: | 306 |
| Population Date: | 2021 |
| Population Footnotes: | [1] |
Thézillieu (in French pronounced as /teziljø/) is a former commune in the Ain department in eastern France. On 1 January 2019, it was merged into the new commune of Plateau d'Hauteville.[2]