Termantia, the present-day locality of Tiermes, is an archaeological site on the edge of the Duero valley in Spain. It is located in the sparsely populated municipio of Montejo de Tiermes (Soria, Castile and León).
During the Iron Age it was a Celtiberian hill fort. It resisted the Romans, and was allied to Numantia during the Celtiberian Wars. It is believed that the Romans gave it the status of municipium. The original name was Romanised as Termes or Termantia. It is remarkable for its impressive site on an arid red sandstone hill and for the way buildings have been carved in the solid rock.