John J. "Jack" Dreyfus, Sr. (August 28, 1913 – March 27, 2009) was an American financial expert and the founder of the Dreyfus Funds.
Born in Montgomery, Alabama, Dreyfus was a graduate of Lehigh University in Pennsylvania. He is widely publicized for being the man who "invented" the commonplace mutual fund through direct marketing to the public.

After selling the fund during the early 1970s, Dreyfus became a major proponent of phenytoin as a means to control nervousness and depression when he received a prescription for Dilantin in 1966. Dreyfus' book about his experience with phenytoin. A Remarkable Medicine Has Been Overlooked, sits on the shelves of many physicians courtesy of the work of his foundation. Despite well over $100 million in personal financing, his push to see phenytoin evaluated for alternative uses has had little lasting effect on the medical community. This was partially due to Parke-Davis's reluctance to invest in a drug nearing the end of its patent life.

His early television commercials featuring a lion emerging from the Wall Street subway station were successful.

According to Barron's Magazine end of Century issue, Jack Dreyfus was considered the 2nd most significant money manager of the last century.

Dreyfus married in 1939 Joan Personette, from whom he was divorced; they had one child, John (Jonny).

His paternal grandfather was a first cousin of Alfred Dreyfus, the protagonist of the French 19th-century anti-Semitic scandal known as the Dreyfus affair.

He also was a high-ranking bridge and gin rummy player, bred champion race horses and won tennis tournaments for players in their 60s and 70s, his foundation said.