The Main is the longest tributary of the Rhine. It rises as the White Main in the Fichtel Mountains of northeastern Bavaria and flows west through central Germany for 525 kilometres to meet the Rhine below Russelsheim, Hesse. The cities of Mainz and Wiesbaden are close to the confluence.

The largest cities on the Main are Frankfurt am Main, Offenbach am Main and Würzburg. It is the longest river lying entirely in Germany (if the Weser-Werra are considered separate).

The Main flows through the north and north-west of the state of Bavaria then across southern Hesse; against the latter it demarcates a third state, Baden-Württemberg, east and west of Wertheim am Main, the northernmost town of that state.

The upper end of its basin opposes that of the Danube where the watershed is recognised as the European Watershed.