Le Grau-du-Roi is south of Aigues-Mortes, in the Gard department. It is in the canton d'Aigues-Mortes, which was originally in the Hérault department, but it was exchanged for the canton de Ganges. It is separated from Aigues-Mortes by a series of saline marshes, called étangs in French, and a bend in the river Vidourle. The étangs are named as follows: l'étang du Ponant, l'étang du Médard, l'étang du Repausset Levant, and the lac de Salonique, which is a lake rather than a marsh, as its name suggests.

To the west of the town, the coastal strip is breached where the Vidourle empties into the Mediterranean, and to the east is that of the Petit Rhône.

The course of these two rivers has been subject to change and reversal. The Rhône used to flow into the étangs further to the west and at one point, the Vidourle flowed into it rather than the sea. The route in the 12th Century was through the Grau Louis and the Old Channel. In 1570, a new breach occurred at a point called Gagne-Petit. It is around this grau that the settlement was formed. The area between the étangs and the sea was settled by Italian immigrants at the end of the 19th century. From then until the 1920s, the economy was based entirely on fishing, but tourism later became important, culminating in the construction of Port-Camargue.

The modern-day resort town benefits from 18 km of fine sand beaches, with sand dunes on the Pointe de Éspiguette forming an important wildlife habitat. The étangs are noted for their birds, which include flamingoes.