The Condes de Castro Guimarães Museum is a museum and library in Cascais, in a summer house on the Santa Marta cove built between 1897 and 1900 for Jorge O'Neill and first called the Tower of São Sebastião. The Count of Castro Guimarães left the house and its collections to the people of Cascais, and the museum opened on 12 July 1931.
The house stands inside Parque Marechal Carmona.

What to see at the Condes de Castro Guimarães Museum
- Building: a summer house designed by the painter Francisco Vilaça, which mixes medieval-style chimneys, fortified towers, pointed windows and Manueline-style arcades.
- Collections: paintings, furniture, porcelain and jewellery left by the Count of Castro Guimarães.
- Chronicle of King Afonso Henriques: a 16th-century manuscript by Duarte Galvão, with illuminations attributed to António de Holanda, kept in the library.
- Music Room: the room where the Count installed an organ in 1912.
- Chapel of São Sebastião: a chapel from about 1594 in the grounds, which gave the house its first name.
- Parque Marechal Carmona: the park around the house, formerly its garden, open to the public.
History of the Condes de Castro Guimarães Museum
- 1897 to 1900: Jorge O'Neill, an aristocrat and financier of Irish descent, builds the Tower of São Sebastião.
- 1910: Manuel Inácio de Castro Guimarães, made Count of Castro Guimarães in 1909, buys the house.
- 1924: the Count leaves the house, its art and its books in his will to the town of Cascais, for a municipal museum and public library.
- 12 July 1931: the museum opens to the public.
Visiting the Condes de Castro Guimarães Museum
- Location: the museum is on Avenida Rei Humberto II de Itália, next to the Santa Marta Lighthouse Museum.
- Opening times: the Bairro dos Museus website publishes current opening times and prices.
Places to visit in Cascais
Cascais, Portugal
Estoril, Lisbon
History of Cascais
Visiting Cascais
Boca do Inferno
Citadel of Cascais
Casa das Histórias Paula Rego
Condes de Castro Guimarães Museum
Santa Marta Lighthouse Museum
Cascais & Guincho
Marina de Cascais