Semnan (sĕmnän´), city (1991 pop. 75,131), capital of Semnan prov., N Iran. It is the trade and transportation center of a fertile agricultural region. Manufactures of the city include textiles, carpets, and fruit. There is a satellite launch facility nearby. Semnan was destroyed by the Oghuz Turks (1036) and by the Mongols (1221). There are many historical remains, including the ruins of several old castles.