The Main is a river in Germany. With a length of (including the White Main: ), it is the longest right tributary of the Rhine, and the longest river lying entirely in Germany (if we consider the Weser and the Werra as two separate rivers; together they are longer). The largest cities along the Main are Frankfurt am Main and Würzburg.

Main rivers are a statutory type of watercourse in England and Wales, usually larger streams and rivers, but also include some smaller watercourses. A main river is defined as a watercourse marked as such on a main river map, and can include any structure or appliance for controlling or regulating the flow of water in, into or out of a main river. The Environment Agency's powers to carry out flood defence works apply to main rivers only. In England main rivers are designated by Defra. The Welsh Government performs this function in Wales. Every other open watercourse in England and Wales is determined by statute as an 'ordinary watercourse'.