Rheinfelden District lies in the northwest of the canton of Aargau in Switzerland, in the Fricktal region. Its capital is Rheinfelden. Around 88% of the population live in the conurbation of Basel. There are 14 municipalities, with a population of 48,164 (as of 31 December 2020) living in an area of 112.09 km2. The population density is around 355 persons per square kilometre.

The Rheinfelden district has an area, as of 2009, of 111.98 square kilometres (43.24 sq mi).