Kalmthout is a municipality in the Belgian province of Antwerp. The municipality comprises the villages of Kalmthout, Achterbroek, Heide, and Nieuwmoer. On January 1, 2007 Kalmthout had a population of 17,508. The total area is 59.45 square kilometres. Kalmthout has a population density of 294 inhabitants per km².
In addition to Kalmthout itself, the municipality also contains the communities of Dorp-Heuvel, Heide, Achterbroek, and Nieuwmoer.
The Arboretum Kalmthout is one of Belgium's most beautiful botanical gardens. The Kalmthoutse Heide is a nature reserve of nearly 10,000 acres spanning the border between Belgium and the Netherlands.