Ondosto was a city in the southern parts of the Forostar on the island of Númenor, on the road that ran westward out of Arandor to Andúnië. It may have been associated with the stone-quarries of Forostar.
Its name is apparently derived from Quenya ondo "stone" and osto "city". Curiously, this would mean its name would be passable as a Quenya name for the ancient elven city of Gondolin. It is not entirely unlikely that the city was named such in its memory.