Tesserarius (from Latin tessera - clay tablet) was a guard commander in the Roman army, also performing administrative functions. The tesserarius was a principalis—a junior officer—and reported directly to the centurion.
Each century had one tesserarius. He was responsible for the reports and records of his unit's guard, which were submitted to senior officers. He also organized the watch duties and managed the storage and transmission of passwords, which were written on tesseras, giving rise to the name of this military rank. The tesserarius received a salary one and a half times higher than that of a regular legionary. During the Republic, according to Polybius, the tesserarius was simply a legionary chosen as a guard for one night, entrusted with the password.
In appearance, the tesserarius likely did not differ much from a regular legionary. A significant number of reliefs have been found where a wax tablet is tucked behind the cingulum of a legionary—this tessera on the relief is probably an indication that the figure depicted is a tesserarius, although there is no direct evidence to confirm this.