William Gregg (died 1708) was a Scottish spy who had been a clerk to the Secretary of State, Robert Harley. He was executed for treason after it was discovered that he had been selling State secrets to the French. The affair gravely damaged Harley politically, as he was rightly criticised for facilitating Gregg's treason through his lax security arrangements, although it is generally accepted that he had no knowledge of the treason itself.