Château de Costaérès, Gothic Revival château on île de Costaérès, France

Château de Costaérès is a four-storey Gothic Revival house built on a one-hectare islet, with a round corner tower, a crenellated stone gateway and slate roofs. Its walls are of pink granite from the La Clarté quarries at Perros-Guirec. A west wing and tower were added at the rear after 1900.

Bruno Abakanowicz, a Lithuanian-Polish mathematician and electrical engineer, bought the one-hectare islet from customs officer René Le Brozec in 1892 at 0.25 franc per square metre, a total of 2,500 francs. Engineer Lanmoniez drew the plans and Pierre Le Tensorer of Lannion led the works, finished in 1896. Abakanowicz died in 1900; his daughter Zofia completed the house and added a west wing.

The name Costaérès comes from the Breton coz-seherez, "old drying place", because fishermen spread their catch to dry in the sun on this islet. The novelist Henryk Sienkiewicz, a friend of Abakanowicz, stayed here in 1898–1899, and a beach at Trégastel is named Plage Quo-Vadis after his novel. The château has long appeared on postcards and in guides of the Côte de Granit Rose.

The islet is private property and lies within the Natura 2000 Côte de granit rose-Sept-Îles protected zone. It can be reached on foot at low tide from Tourony beach, along the GR 34 long-distance path; at high tide the channel cuts it off. The château itself is not open to visitors.

The interior woodwork partly comes from the Maurice, a three-masted ship wrecked on the island in a storm in the winter of 1896, while the château was being built. The vessel's timber was recovered and reused inside the house.