William Sinclair, 1st Earl of Caithness

William Sinclair (1410–1484), 1st Earl of Caithness (1455–1476), 3rd Earl of Orkney (1434–1470), Baron of Roslin was a Scottish nobleman and the builder of Rosslyn Chapel, in Midlothian.

He was the grandson of Henry Sinclair, 1st Earl of Orkney and son of Henry Sinclair, 2nd Earl of Orkney, for a time protector of the young James Stewart, the later James I of Scotland. He was Lord High Admiral of Scotland, and was Lord Chancellor of Scotland from 1454 to 1456. He became the first Lord St. Clair in Scotland in 1449.

His father Henry died in 1420. William travelled to Copenhagen in 1422 to establish his claim to the earldom of Orkney, but was at first unsuccessful. Instead, David Menzies was to rule as William's guardian until he came of age. In 1424, William succeeded in wresting control of the earldom from his guardian, however, he was only recognized as Earl of Orkney by King Eric in 1434. After the death without issue of King Christopher of Norway in 1448, Earl William was mentioned as a possible candidate for the vacant Norwegian throne, as the Earl of Orkney was the highest ranking nobleman in Norway and as such held a place in the Norwegian line of succession. However, there are no indications that he pursued this claim.

He made several big territorial transactions during his life.

The first important one was the exchange of his inherited lordship of Nithsdale to the estates of the earldom of Caithness - which soon led to his obtaining the title of Earl in the peerage of Scotland.

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