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'''Ĳssel''' (t. ''Gelderse Ĳssel'') – rzeka w środkowej Holandii (prowincje Geldria i Overijssel). Długość – 120 km, średni przepływ – 300 m³/s.

Ĳssel jest jednym z ujściowych ramion Renu. Oddziela się od Renu pod Arnhem i płynie na północ. Przepływa m.in. przez Zutphen, Deventer i Zwolle, po czym koło miasta Kampen uchodzi do Zwarte Meer – odnogi sztucznego jeziora Ĳsselmeer.

Do czasów średniowiecza bieg Ĳssel był dłuższy – rzeka uchodziła do Morza Północnego w miejscu zwanym Vlie (Vliestroom) pomiędzy dzisiejszymi wyspami Vlieland i Terschelling. Później, do czasu budowy wielkiej tamy Afsluitdijk i utworzenia sztucznego jeziora Ĳsselmeer Ĳssel uchodził do zatoki Zuiderzee.

Bifurkacja Ĳssel od Renu jest prawdopodobnie sztuczna. Pierwotnie dzisiejszy Ĳssel był samodzielną rzeką – dalszym ciągiem niewielkiej rzeki Druzusa Starszego jako rubież obronna przeciw plemionom germańskim i jako wojskowa droga wodna. Od tej pory Ren stał się głównym źródłem wody dla Ĳssel, jednak w porównaniu do pozostałych ramion Delty Renu rzeka zachowała pewną odrębność – przyjmuje dopływy (Berkel, Schipbeek) i buduje swoją własną deltę, a ilość wód z dopływów wystarczyłaby do jej samodzielnej egzystencji.

{{Geobox River
<!-- *** Name section *** -->  
| name                        = IJssel
| native_name                 = 
| other_name                  = Gelderse IJssel
| other_name1                 =
<!-- *** Image *** --->
| image                       = Deventer zonsondergang k.JPG
| image_size                  = 220
| image_caption               = Sunset on the IJssel at Deventer
<!-- *** Country etc. *** -->
| country                     = The Netherlands
| country1                    =
| state                       = Gelderland
| state1                      = Overijssel
| region                      = Liemers
| region1                     = Veluwe
| region2                     = Achterhoek
| region3                     = Salland
| district                    = 
| district1                   = 
| city                        = Arnhem (suburbs)
| city1                       = Doesburg
| city2                       = Zutphen
| city3                       = Deventer
| city4                       = Zwolle (suburbs) 
| city5                       = Kampen
| state_type                  = Province
<!-- *** Geography *** -->
| length                      = 125
| watershed                   = 
| discharge_location          = 
| discharge_average           = 
| discharge_max               = 
| discharge_min               = 
| discharge1_location         = 
| discharge1_average          = 
<!-- *** Source *** -->
| source_name                 = Nederrijn
| source_location             = Westervoort
| source_district             = 
| source_region               =
| source_state                = Gelderland
| source_country              = The Netherlands
| source_lat_d                = 
| source_lat_m                = 
| source_lat_s                = 
| source_lat_NS               = 
| source_long_d               = 
| source_long_m               = 
| source_long_s               = 
| source_long_EW              = 
| source_elevation            = 
| source_length               = 
<!-- *** Mouth *** -->
| mouth_name                  = Ketelmeer (inlet of lake IJsselmeer)
| mouth_location              = Kampen
| mouth_district              = 
| mouth_region                = Salland
| mouth_state                 = Overijssel
| mouth_country               = The Netherlands
| mouth_lat_d                 = 
| mouth_lat_m                 = 
| mouth_lat_s                 = 
| mouth_lat_NS                = 
| mouth_long_d                = 
| mouth_long_m                = 
| mouth_long_s                = 
| mouth_long_EW               =
| mouth_elevation             = 
<!-- *** Tributaries *** -->
| tributary_left              = 
| tributary_left1             = 
| tributary_right             = Oude IJssel
| tributary_right1            = Berkel
| tributary_right2            = Schipbeek
<!-- *** Free fields *** -->
| free_name                   =  
| free_value                  = 
<!-- *** Map section *** -->
| map                         = Location IJssel.PNG
| map_size                    = 220
| map_caption                 = Location of river IJssel in dark blue.
}}
Dutch German-Dutch border, the other two being the rivers Nederrijn and Waal.

==History==
The name "IJssel" is thought to derive from the Germanic ''i sala'', meaning "dark water". A branch near its mouth (between Zwolle and Kampen) is today called ''Zwarte Water'', Dutch for "black water". In medieval times, the IJ (near Amsterdam) once were branches of river IJssel.

===The IJssel as the lower part of the Oude IJssel===

The IJssel was the lower part of the small river German ''Issel''),  that rises in Germany and is now a 70&nbsp;km long tributary of the IJssel. The connection between Rhine and IJssel was probably artificial, allegedly dug by men under the Roman general Nero Claudius Drusus as a defence against Germanic tribes and to let Roman ships carry troops along it<ref></ref>. The current Oude IJssel is the second-largest contributor to the flow of the river until today, after river Rhine.

The source of the Oude IJssel is near Borken in North Rhine-Westphalia, Germany. First it flows south-west until it nearly reaches the Rhine near Wesel; then it turns north-west. After passing through Isselburg it crosses the border with the Netherlands. The river, called Oude IJssel from here, then flows through Doetinchem and joins the IJssel at Doesburg.

==Characteristics==
The average discharge of the IJssel can change significantly. The average discharge has been stated as 300 cubic meters per second. This can be as low as 140 and as high as 1800, depending on the Nederrijn locks west of Arnhem, which regulate the amount of water taken in by the Nederrijn and IJssel branches.

As a lowlands river, the IJssel has a lot of bends and naturally occurring dead branches (locally called "hank"); some bends have been cut off by man (most notably near Meuse. The naturally occurring phenomenon of sedimental island-forming in the outside of bends has been regulated to the point of non-occurrence since the late nineteenth century.

===The IJssel as a Rhine distributary===
From the moment the connection between the Rhine and IJssel was dug, the Rhine became the largest contributor to the flow of the IJssel, although only a relatively small amount of the total Rhine flow makes its way into the IJssel system. Various tributaries can sometimes add a considerable volume of water to the total flow of the IJssel, such as the Rhine delta that takes up tributary rivers rather than giving rise to distributaries. 

thumb|left|The IJssel delta (click on image to read legend).Only in the last few miles of the river's run, near the city of Kampen, distributaries form, resulting in the relatively minor IJssel delta. Some of the branches have been dammed up to lower the risk of flooding; others have silted up. Several of the delta branches are, however, still connected without interruption. Most of the damming-up was done prior to 1932, when the Zuiderzee was turned into the freshwater IJsselmeer lake. The area had been prone to flooding in times of northwestern gales, pushing back the saline Zuiderzee water into the IJssel delta and preventing the water flowing from the mouths of the IJssel branches from discharging into the Zuiderzee.

The modern-day names of the delta branches are, west to east, Keteldiep, Kattendiep, Noorddiep, Ganzendiep and Goot. Of these, the Keteldiep and Kattendiep channels are the main navigational arteries; the Noorddiep has been closed off at both sides. Another branch, De Garste, had already completely silted up by the middle of the nineteenth century<ref name="histo"> </ref>. Until the beginning of the twentieth century, the Ganzendiep branch up to the Goot fork was known as IJssel proper<ref name="histo" /> due to it being the historical main channel, with the present-day main channel being called Regtediep or Rechterdiep until well into the twentieth century<ref></ref>.

The IJssel, although now technically "merely" a Rhine branch, has retained most of the character of a distinct river in its own right, in no small part due to the fact that it has its own tributaries and, in river Oude IJssel, even a former headstream.