The former public convenience in North Street, Dowlais, was up for auction starting at £6,000 but ended up being sold for £37,000.
Auctioneers Paul Fosh said the building, which had been owned by the council, could be turned into a cafe, shop or even a new home.
A spokeswoman said there had been "a lot of interest" in the property.
Disused public toilets in Merthyr Tydfil have been sold for more than six times its guide price.