The gens Tertia was an obscure plebeian family at ancient Rome.
The nomen Tertius is derived from the Latin for "third", which was used as a cognomen from the earliest period of Roman history.
While it may anciently have been a praenomen corresponding with similar masculine names, such as Quintus, Sextus, and Decimus, only the feminine form, Tertia, appears to have been in use during the Republic, and only in imperial times does the masculine form appear, rarely, as a praenomen.
[1] The main praenomina of the Tertii seem to have been Titus, Aulus, Lucius, Marcus, and Publius.
The filiation of one early member includes the Oscan praenomen Herius.