PSDN is an acronym for Public Switched Data Network, a publicly-available network supporting packet-switched data, separate from the PSTN.

In the UK, this term refers only to PSS (Packet Switch Stream), an X.25-based packet-switched network, originally used to provide leased-line Internet connections.

Although superficially similar to the PSDN, ISDN, ADSL, SDSL, VDSL are not examples of it. ISDN is a circuit-switched digital network, and DSL variants are packet-switched data services overlaid on the PSTN.