Symmetric Digital Subscriber Line (SDSL) is a Digital Subscriber Line (DSL) variant with data rates of 72 kbit/s to 2.3Mb/s. It runs over one pair of copper wires, with a maximum range of about 3 kilometers or 1.86 miles.

The main difference between ADSL and SDSL is that SDSL has the same upstream data rate as downstream (symmetrical), whereas ADSL always has smaller upstream bandwidth (asymmetrical). However, unlike ADSL, it can't co-exist with a conventional voice service on the same pair as it takes over the entire bandwidth. It is mainly targeted at small and medium businesses who may host a server on site, (eg a Terminal Server or Virtual Private Network) and want to use DSL, but do not need the higher performance of a leased line.