The IJssel (Dutch: [ˈɛisəl] (listen); Dutch Low Saxon: Iessel(t) [ˈiːsl̩(t)]) is a Dutch distributary of the river Rhine that flows northward and ultimately discharges into the IJsselmeer (before the 1932 completion of the Afsluitdijk known as the Zuiderzee), a North Sea natural harbour. It more immediately flows into the east-south channel around the Flevopolder, Flevoland which is kept at 3 metres below sea level. This body of water is then pumped up into the IJsselmeer.

It is sometimes called the Gelderse IJssel (IPA: [ˌɣɛldərsə ˈʔɛisəl] (listen); "Gueldern IJssel") to distinguish it from the Hollandse IJssel. It is in the provinces of Gelderland and Overijssel. The Romans knew the river as Isala. It flows from Westervoort, on the east side of the city of Arnhem.

Similar to the Nederrijn which shares its short inflow, the Pannerdens Kanaal, it is a minor discharge of the Rhine. At the fork where the Kanaal is sourced the Rhine becomes named the Waal. This splitting-off is west of the German border. The Waal in turn interweaves with other rivers and the lower course of the Nederrijn, which altogether is known as the Rhine–Meuse–Scheldt delta.

The name contains the digraph ij, used throughout modern Dutch orthography, which is why both letters appear capitalized (as in: IJmuiden and IJsselmeer).