William Rawlinson Marsh (19 June 1917 – 19 December 1995) was a British theatrical agent. Widely known as the "doyen of theatrical agents", he was the founder and namesake of Billy Marsh Associates, a renowned entertainment agency, in recent times representing the likes of Jon Culshaw, Esther Rantzen and Fiona Phillips. He is the only theatrical agent ever to be subject of This Is Your Life, surprised by Michael Aspel in 1990.