Grez-Doiceau is a municipality of Wallonia 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 square kilometres (21.41 sq mi) which gives a population density of 224 inhabitants per km2. Grez-Doiceau is watered by the Train, a tributary of the Dyle.