John Cassidy may refer to:
- John Cassidy (artist) (1860–1939), Irish sculptor and painter
- John Cassidy (baseball) (1855–1891), American baseball player
- John Cassidy (basketball) (born 1947), Canadian Olympic basketball player
- John Cassidy (journalist) (born 1963), business journalist and published author
- John Cassidy (magician) (born 1967), American magician and balloon artist
- John Cassidy (author), American author and publishing executive
- John Cassidy (chancellor), Australian businessman and former university chancellor
- John Cassidy (seismologist) (born 1959), Canadian seismologist
- John Cassidy (speed skater) (born 1952), Canadian Olympic speed skater
- Jack Cassidy (1927–1976), American actor
- John Edward Cassidy (1896–1984), American lawyer; Illinois Attorney General
- John E. Cassidy Jr (1924—2003), member of the Illinois House of Representatives.
- John F. Cassidy, Vice Chairman of the board and former Chief Executive Officer of Cincinnati Bell Inc.
- John Joe Cassidy (died 1995), Gaelic football player for Cavan
- John P. Cassidy (1912–?), Los Angeles City Council member, 1962–1967