Kastellholmen is a small island in the centre of Stockholm, Sweden, connected to the adjacent Skeppsholmen through the bridge Kastellholmsbron. It has an area of 31,000 m².
On the island there is a small castle, Kastellet, which was built by Fredrik Blom. Kastellholmen has previously been known as Notholmen, Lilla Beckholmen and Skansholmen. Kastellholmen is part of the national city park Ekoparken.