The Castell d'Onda is a fortification located in the town of Onda (Plana Baixa, Valencian Country). In 1967 it was declared a Historic-Artistic Complex and a Cultural Property.

The original structure was built by Muslims in the x century on top of an old Roman settlement, although the original structure appears buried under different reconstructions carried out over the centuries. It came to be known for its great size as the "castle of the 300 towers", since it was said to have as many towers as there are days in the year. It has been in the process of restoration for a few years. It currently houses a museum, which highlights the exhibition of Muslim plasterwork, from a palatine house from the beginning of the xiii century.