It is a common inhabitant of Texas and Florida estuaries, but was later introduced all over the world.

[5] R. harrisii was first discovered in Europe in the Zuider Zee, the Netherlands, and is now also found in Denmark, Belgium, Germany, France, Poland, Estonia, Finland, Russia, Portugal, Spain, Italy, Romania and Bulgaria, from the Black Sea and Caspian Sea.

[2][6][7] In the British Isles, R. harrisii has only been observed in Roath Docks, Cardiff, which have lower salinity (12‰) than the surrounding waters.

Breeding freshwater populations have been found in the Brazos River basin in Texas, notably the Possum Kingdom State Park and Lake Granbury.

[8] Its presence in the Baltic Sea is blamed for a negative ecosystem shift.