Lews Castle (Scottish Gaelic: Caisteal Leòdhais) is a Victorian era castle located west of the town of Stornoway, Isle of Lewis, Scotland. It was built in the years 1847-57 as a country house for Sir James Matheson who had bought the whole island a few years previously with his fortune from the Chinese Opium trade. It was designed by the Glasgow architect Charles Wilson.

In 1918, the Lewis estate including the castle was bought by industrialist Lord Leverhulme from the Matheson family. He gifted the castle to the people of Stornoway parish in 1923.

During World War II the Castle was taken over as accommodation for air and ground crew of 700 Naval Air Squadron, who operated a detachment of six Supermarine Walrus aircraft from a slipway at Cuddy Point in the Grounds. The base was referred to as HMS Mentor. It was also used for accommodation for students of Lews Castle College in the 1950s.

Today the building is owned by the local council, Comhairle nan Eilean Siar, and is protected as a category A listed building. Lews Castle was awarded £4.6 million by the Heritage Lottery Fund, on 22 November 2011, to enable it to be converted into a bilingual museum and cultural centre.