Thurso Castle was a 17th century probable castle belonging to the Sinclairs, some remains of which were incorporated into a 19th century replacement.

Some sources conflate this castle with the Castle of Ormlie which is thought to have been the original castle of Thurso, making any pre-17th century history of this castle difficult to establish.

Most sources agree that this castle, also known as Thurso East, was built around 1660 for George Sinclair, 6th Earl of Caithness. He contracted the master mason Donald Ross at a cost of 600 merks. Whether or not it had defensive features is unclear.

In 1676 Sinclair died at the castle without issue and heavily in debt. Before his death he had transferred his estates and title to his principal creditor, John Campbell of Glenorchy, who also married Sinclair’s widow, Mary Campbell, daughter of Archibald Campbell, 1st Marquess of Argyll. The following year Campbell was created Earl of Caithness by king’s patent which didn’t go down well with the wider Sinclair family.

A group of Sinclairs, led by George Sinclair of Keiss, son of the late Francis Sinclair of Northfield, and grandson of the 6th Earl, attacked and partially demolished Thurso-East. Campbell took legal action against the attackers however in 1681 George Sinclair’s right to the earldom was recognised.

The castle was remodelled in the 18th century by Sir John Sinclair, 1st Baronet, author of the Statistical Account of Scotland, and again in 1806 and 1834 when it was enlarged by William Burn. To the south-west of the four storey main block was a massive five storey square tower built in the style of an old keep, with a small three storey square tower projecting up from the re-entrant angle.

Around 1872 much of the old castle is said to have been demolished and a new mansion was built in a French chateau style for Sir Tollemache Sinclair to a design by David Smith.

During the Second World War an anti-ship mine came ashore and exploded on the beach below the castle, causing damage to some windows. In 1951 there was a fire and the castle was abandoned and the roof removed. The following year some unstable sections of the castle were demolished.

Thurso Castle is still owned by the Sinclair family and is the seat of Viscount Thurso of Ulbster.