Rheinfelden (Swiss German: Rhyfälde, [ˈɾiːfæld̥ə]) is a municipality in the canton of Aargau in Switzerland, seat of the district of Rheinfelden. It is located 15 kilometres east of Basel. The name means the fields of the Rhine, as the town is located on the Hochrhein. It is home to Feldschlösschen, the most popular beer in Switzerland. The city is across the river from Rheinfelden in Baden-Württemberg; the two cities were joined until Napoleon Bonaparte fixed the German–Swiss border on the Rhine in 1802 and are still socially and economically tied.

The old town of Rheinfelden lies on the left bank of the Rhine, where the river is divided into two arms by the "Inseli", a roughly 150-metre-long island. This is on the verge of a tectonic plate, the Rhine rift. A huge vortex, "St-Anna-Loch" tears at this point, with water up to fifty metres in depth. Nearly 400 metres east is the Magdenerbach. The Rhine is navigable by ship from Rheinfelden all the way to the North Sea.

Around the city center stretches a large gravel plain. While this stretches just one kilometre wide to the West, it extends to the east to the moraine of Möhlin with a width of around three kilometres; in the south, the plain is limited through the wooded, gently-rising foothills of the Tafeljura. These are the Steppberg (373 m above sea level) and the "Berg" (419 m), both in the south-east. Between these two hills lie the deeply incised valleys of the Magdenerbach.