Acton Castle is a small castellated mansion near Perranuthnoe, Cornwall. It is a Grade II* listed building. It was built c. 1775, and according to some sources around 1790, by John Stackhouse of Pendarves, who was a distinguished botanist with an interest in seaweed and plants mentioned by Theophrastus. Stackhouse constructed the castle, with the main purpose of studying marine algae. The primary material used for the construction is granite, with the facade and the chimneys made of dressed granite. It has a grouted roof with walls topped by embattled parapets. Wings of two storeys, with tripartite windows, were added at the beginning of the 20th century during its conversion to a country hotel.