Henry Sinclair, 1st Earl of Orkney, Baron of Roslin, and Lord of Shetland (c.1345-c.1400), was a Scottish nobleman. He is sometimes identified by the alternate spelling Henry St Clair. He was the grandfather of William Sinclair, the builder of Rosslyn Chapel. He is also noted for being the subject of speculation that he undertook early explorations of Greenland and North America in about the year 1398. According to a biography published many years after his death, he died in battle against the English around the year 1400.

Little is known about Sinclair's life. Much has been written through conjecture, however, about his possible career as an explorer. In particular, starting in the 19th century, he was identified by one historian 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 not discovered and published until the 16th century), the exact course of the voyage, as well as whether or not it even occurred, has not been firmly established. Many historians regard the letters (and the accompanying map) as a hoax, either by the Zeno brothers or by the descendant who later published them. Moreover, the identification of Zichmni as Henry Sinclair is very controversial, although it is often taken for granted among supporters of the theory.

The most controversial theories speculate that Henry (Zichmni) traveled not only to Greenland but to present-day Nova Scotia, where he may have founded a settlement among the Micmac Indians, and perhaps as far south as present-day Massachusetts and Rhode Island. According to these theories, he may have been responsible for the building of the Newport Tower.

The theory that Henry Sinclair explored North America is based on three separate propositions:

1. That the letters and map ascribed to the Zeno brothers and published in 1558 are authentic.
2. That the voyage described in the letters taken by Zichmni around the year 1398 actually reached North America.
3. That the Zichmni is Henry Sinclair.

External sources

• Brief biography with slant towards belief in his explorations
• Report of 1997 symposium delaring the 1398 expdeition Not Proven
• Refutation of Henry-as-Zichmni theory