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 which gives a population density of 224 inhabitants per km<sup>2</sup>. Grez-Doiceau is watered by the Train, a tributary of the Dyle.
The municipality consists of the following districts: Archennes, Biez, Bossut-Gottechain, Grez-Doiceau, and Nethen.
The Nobel Prize-winning cytologist and biochemist Christian de Duve (1917âÂÂ2013) died in Grez-Doiceau.