Kastellholmen (The Castle Isle) is an islet in the centre of Stockholm, Sweden.
It is connected to adjacent Skeppsholmen through the Kastellholmsbron bridge.
Kastellholmen has previously been known as Notholmen, Lilla Beckholmen and Skansholmen.
On the island there is a small castle, Kastellet, which was built between 1846 and 1848 under design by Swedish officer and architect Fredrik Blom (1781–1853).
This article about a location in Stockholm County, Sweden is a stub.