Piaseczno County is a unit of territorial administration and local government (powiat) in Masovian Voivodeship, east-central Poland. It came into being on January 1, 1999, as a result of the Polish local government reforms passed in 1998. Its administrative seat and largest town is Piaseczno, which lies south of Warsaw. The county contains three other towns: Konstancin-Jeziorna, east of Piaseczno, Góra Kalwaria, south-east of Piaseczno, and Tarczyn, south-west of Piaseczno.