Grez-Doiceau (Dutch: Graven) is a Walloon municipality located in the Belgian province of Walloon Brabant. On January 1, 2006 Grez-Doiceau had a total population of 12,403. The total area is 55.44 km² which gives a population density of 224 inhabitants per km². Grez-Doiceay is watered by the Train, a tributary of the Dyle.