Sir John Liddell, KCB, FRS was a Scottish medical doctor who served as Director-General of the Medical Department of the Royal Navy, and senior medical officer of the Royal Hospital at Greenwich.

Greenwich Hospital was a permanent home for retired sailors of the Royal Navy, which operated from 1692 to 1869. Its buildings, in Greenwich, London, were later used by the Royal Naval College, Greenwich and the University of Greenwich, and are now known as the Old Royal Naval College. The word "hospital" was used in its original sense of a place providing hospitality for those in need of it, and did not refer to medical care, although the buildings included an infirmary which, after Greenwich Hospital closed, operated as Dreadnought Seaman's Hospital until 1986.