Leithen Water (Scottish Gaelic: Leitheann / Abhainn Leitheinn) is a tributary of the River Tweed in Scotland. It rises in the Moorfoot Hills and joins the Tweed near the town of Innerleithen, whose name comes from the Scottish Gaelic inbhir, meaning a confluence, and anglicised as "inner" or "inver". The Brythonic equivalent is "Aber".