Tertius may refer to:
People Tertius of Iconium, first century Christian martyr, saint and bishop Tertius, fifth century Christian martyr and saint (see Denise, Dativa, Leontia, Tertius, Emilianus, Boniface, Majoricus, and Servus) Tertius Bosch (1966–2000), South African cricketer Tertius Chandler (1915–2000) American historian and author Tertius Delport, South African politician Tertius Losper (born 1985), Namibian rugby union fullback T. Tertius Noble (1867–1953), English-born organist and composer Tertius Rebelo (1915–1976), Brazilian politician Tertius Zongo (born 1957), former Prime Minister of Burkina Faso
Other uses Tertius (law), a term in contract law referring to an interested third party Tertius, the underworld of Pluto (mythology) Tertius Lydgate, a main character in George Eliot's novel Middlemarch Tertius, a planet in Robert A . Heinlein's science fiction novel Time Enough for Love and subsequent books featuring Lazarus Long Tertius (sandbank), a German sandbank in Meldorf Bay
See also Alexander Monro (tertius), named "Tertius" to distinguish him from his grandfather and father Tertia (disambiguation), the feminine form of Tertius