Tertius may refer to:

• 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

• 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

• Alexander Monro (tertius), named "Tertius" to distinguish him from his grandfather and father
• Tertia (disambiguation), the feminine form of Tertius