Marnaz (; Savoyard: Marnâ) is a commune in the Haute-Savoie department in the Auvergne-Rhône-Alpes region in south-eastern France. It is around 34 km south-east of Geneva.
As with many polysyllabic Arpitan toponyms or anthroponyms, the final -x marks oxytonic stress (on the last syllable), whereas the final -z indicates paroxytonic stress (on the penultimate syllable) and should not be pronounced, although in French it is often mispronounced due to hypercorrection.
Marnaz is twinned with Quincinetto, Italy since 1996.