John Dennis may refer to:
- John Dennis (1771–1806), Maryland congressman
- John Dennis (1807–1859), his son, Maryland congressman
- John Dennis (bishop) (1931–2020), former Bishop of St Edmundsbury and Ipswich
- John Cawsey Dennis (1871–1939), English carmaker, one of the two brothers who founded Dennis Brothers
- John Dennis (diplomat) (born 1959), British diplomat, U.K. Ambassador to Taiwan, and former U.K. Ambassador to Angol
- John Dennis (dramatist) (1658–1734), English dramatist
- John Dennis (Missouri politician) (1917–2000), Missouri politician
- John Dennis (ornithologist) (c. 1916 – 2002), American ornithologist
- John Dennis (talk show host) (born c. 1952), American radio talk show host
- John B. Dennis (1835–1894), American Union Civil War era brevet brigadier general
- John E. Dennis (born 1939), mathematician
- John N. Dennis (born 1933), American politician in the New Jersey General Assembly
- John Stoughton Dennis (1820–1885), Canadian surveyor
- John William Dennis, Member of Parliament for Birmingham Deritend, 1918–1922a