Diest (Dutch pronunciation: [ˈdist] ⓘ) is a city and municipality located in the Belgian province of Flemish Brabant.

The municipality comprises the city of Diest proper and the towns of Deurne, Kaggevinne, Molenstede, Schaffen and Webbekom.

Between 1499 and 1795 the town was controlled by the House of Nassau (as were Breda in the Netherlands, Dillenburg in Germany and Orange in France) which was also the family of the Princes of Orange who at the end of the Napoleonic Wars became in 1815 the kings and queens of the Netherlands after the termination of the Dutch republic at the hands of revolutionary forces in 1795.

His son Philip William - who unlike his father remained a pious Catholic throughout his life - is buried in Diest.

In his will Philip William commanded that the city's parish church of Saint Sulpice should celebrate a yearly Requiem Mass for his soul.

Exhibits in the museum include the armour of Philip of Orange and a portrait of René of Orange-Nassau and his wife Anna of Lorraine.

The whole structure is purely geared towards military functionality, with the only embellishment being the bluestone gate in neoclassical style.

The Germans used the citadel during World War II, and after the surrender, the Allies used it for a short time as a prison.

Together with the other parts of the Diestsesteenweg defence works, Fort Leopold, the Schaffensepoort, and the guard-locks of the Zichem and Leuven gates, it provides a good idea of the military architecture of the second quarter of the 19th century.

This new building were projected to be ready by 2023 in an agreement in principle between "stad Diest" and the "vzw Vereniging Diestse Ziekenhuizen".