Jacalyn M. Duffin CM FRSC (born 1950) is a Canadian medical historian and hematologist.
She held the Hannah Chair, History of Medicine at Queen's University from 1988 until 2017.
Soon after this, she moved to Paris, where she elected to study hematology and René Laennec at the Sorbonne.
Upon her return to Canada, Duffin settled in Ottawa, where she took on a contract to review a set of slides, which she assumed were to be used in a malpractice suit.
Duffin's testimony was to be used by the Vatican to determine whether Marie-Marguerite d'Youville (1701–1771) had performed a miracle and was worthy of canonization.