Oud Poelgeest is a castle in Oegstgeest, north of Leiden, that was the former home of the Dutch scientist Herman Boerhaave (1668–1738).

The castle was built in 1668 on the foundations of an earlier construction, in the then popular Dutch Neoclassical style.

[2] Boerhaave bought the castle in 1724[3] and designed the garden for his large botanical collection that no longer fit in the Hortus Botanicus Leiden.

This site was renowned during his lifetime and rivaled Hortus Cliffortianus, the garden of his friend and sponsor to Linnaeus.

The onion domes were added after Herman Boerhaave's death by a later owner in 1866, along with the coach house (also a monument) in 1867.