Nisa (Ancient Greek: Νῖσος, Νίσα, Νίσαιον; Turkmen: Nusaý; also Parthaunisa) was an ancient settlement of the Parthians, located near the Bagyr neighborhood of Ashgabat, Turkmenistan, 18 km west of the city center. Nisa is described by some as the first seat of the Arsacid Empire. It is traditionally assumed to have been founded by Arsaces I (reigned c. 250 BC–211 BC) and was reputedly the royal residence of the Parthian kings, although it has not been established that the fortress at Nisa was either a royal residence or a mausoleum.

In 2007, the fortress was declared a World Heritage Site by UNESCO.