Henry I Sinclair
Conte di Orkney e barone di Roslin

Henry I Sinclair, Earl of Orkney and feudal baron of Roslin (c. 1345 – c. 1400), was a Scottish nobleman. He is sometimes identified by another spelling of his surname, St. Clair. He was the grandfather of William Sinclair, 1st Earl of Caithness, the builder of Rosslyn Chapel. He is also noted for the modern legend that he undertook early explorations of Greenland and North America in about the year 1398.

Henry was son and heir of William Sinclair, Lord of Roslin, and Isobel of Strathearn, a daughter of Maol Ísa, Earl of Orkney. Henry's maternal grandfather had been deprived of much of his lands (the earldom of Strathearn being completely lost to the King of Scots). On his father's death, sometime after 13 Sep 1358 (when he was yet living), Henry succeeded as baron of Roslin, Pentland and Cousland, a group of minor properties in Lothian. The Sinclair Diploma states that Henry married Joneta (or Joan) Halyburton, daughter of Walter, Lord of Dirleton, and that they had a son Henry who became the next Earl of Orkney. Also apparently Elizabeth Sinclair, wife of the Justiary John Drummond of Cargill was a daughter of this Henry.

Three cousins: Alexander de L'Arde, Lord of Caithness; Malise Sparre, Lord of Skaldale; and Henry, were rivals for the succession to the earldom of Orkney. On August 2, 1379 at Marstrand, near Třnsberg, Norway, King Haakon VI of Norway invested and confirmed Henry as the Norwegian Earl of Orkney over a rival claim by his cousin Malise Sparre.

In 1389, Henry attended the coronation of King Eric of Pomerania in Norway, and pledged his oath of fealty. Historians have speculated that in 1391, Earl Henry and his troops slew Malise Sparre near Scalloway, Tingwall, Shetland.

There is no firm evidence on when Henry died. The Sinclair Diploma, written or at least commissioned by his grandson states: "...he retirit to the parts of Orchadie and josit them to the latter tyme of his life, and deit Erile of Orchadie, and for the defence of the country was slain there cruellie by his enemiis..." We also know that sometime in 1401: "The English invaded, burnt and spoiled certain islands of Orkney." The assumption is that Henry either died in this invasion, or was already dead.

Little else is known about Sinclair's life. Much has been written through conjecture, however, about his possible career as an explorer. In 1784, he was identified by Johann Reinhold Forster as possibly being the Prince Zichmni described in letters allegedly written around the year 1400 by the Zeno brothers of Venice, in which they describe a voyage throughout the North Atlantic under the command of Zichmni.

The authenticity of the letters (which were allegedly rediscovered and published in the early 16th century), the exact course of the voyage, as well as whether or not it even occurred, are challenged by historians. Most regard the letters (and the accompanying map) as a hoax by the Zenos, who published them. Moreover, the identification of Zichmni as Henry Sinclair is very controversial, although it is taken for granted among supporters of the theory.

The most controversial contentions speculate that Sinclair traveled not only to Greenland but also to present-day Nova Scotia, where he founded a settlement among the Micmac indigenous people. The evidence that has been claimed to support this voyage consists of the Micmac flag, which has been claimed to be a reversed sailing flag of the Knights Templar; an Italian-made cannon found in Louisbourg Harbour, Nova Scotia[citation needed] now housed in Fortress Louisbourg that was made in Italy before single-cast forging of cannons was invented; and specific interpretations of the Newport Tower and the carving of the Westford Knight.

This is related to the contention that there are stone carvings of American plants in Rosslyn Chapel in Scotland. The Chapel was built by Henry Sinclair's grandson William Sinclair and was completed in 1486. Columbus made his first voyage in 1492. This is seen by authors Christopher Knight and Robert Lomas as being compelling evidence for the theory that Henry sailed to America, although scholars have put forth the interpretation that the plants are stylised depictions of common European plants.