Reverend William Sinclair (died 1830) was an Irish Presbyterian minister and, as a radical democrat, a member of the Society of United Irishmen.
[2][3] On 14 October 1791, he was one of 12 men including Henry Joy McCracken and Wolfe Tone who met to form the Belfast Society of the United Irishmen.
He fell under the suspicion of Lord Castlereagh, his congregant, former student and son and heir to the leading landowner in north Down, the Earl of Londonderry.
Afterwards "we had a very jolly dinner: Cleland quite drunk, Sinclair considerably so, my father not a little, others lying heads and points, the whole very happy, and ‘God Save the King’ and ‘Rule Britannia’ declared permanent.
After the arrest of Reverend William Steel Dickson on the eve of the Battle of Ballynahinch in June 1798, his brother George Sinclair was briefly the declared Adjutant General of the United forces of County Down.