William Rawlinson "Billy" Marsh (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 the BBC biographical documentary series This Is Your Life.