Nisa (Ancient Greek: Νῖσος and Νίσα and Νίσαιον; also Parthaunisa, Turkmen: Nusaý) 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.
Nisa was a major trading hub in the Parthian Empire. Nisa was later renamed Mithradātkert Parthian: 𐭌𐭕𐭓𐭃𐭕𐭊𐭓𐭕 ("fortress of Mithradates") by Mithridates I of Parthia (reigned c. 171 BC–138 BC). The region was famous for the fast and beautiful horses.
Nisa was totally destroyed by an earthquake, which occurred during the 1st decade BC.