Dr Lancelot Priestley is a fictional investigator in a series of books by John Rhode.
After 1924 Dr Priestley took over from Dr Thorndyke as the leading fictional forensic investigator in Britain, and featured in over 50 novels written over 40 years, solving many ingenious and misleading murders, usually at the request of the police but only if the case piques his scientific curiosity, having little interest in criminal justice. In the early books Dr Priestly provides assistance mainly to his friend Chief Inspector Hanslet. In later books Dr Priestley becomes an armchair detective and the bulk of the legwork is done by Superintendent Jimmy Waghorn of Scotland Yard and his secretary and companion, Harold Merefield. Harold Merefield is engaged to Dr Priestly's daughter April. Chief Inspector Hanslet (now retired) appears in several later works as a dinner guest of Dr Priestly passing on his professional wisdom to Waghorn.