The name is probably derived from a Silesian word meaning "wet swampy place". In a papal bull from Hadrian IV in 1155, the river is called the Selenza.
The names of the Ślęza and Mount Ślęża are both of Silesian origin, but the Ślęza is spelled with a standard z and Mount Ślęża is spelled with a ż, pronounced like an Englishzh.