A tesserarius (Latin: tesserārius, from tessera, a small tile or block of wood on which watchwords were written), was a watch commander in the Roman army. They organized and had command over the nightly guard assigned to keep watch over the fort when in garrison or on campaign and were responsible for getting the watchwords from the commander and seeing that it was kept safe. There was one tesserarius to each centuria (Wilkes, 1972). They held a position similar to that of a non-commissioned officer in modern armies and acted as seconds to the optiones.
Tesserary pay was one and a half times (sesquiplicarii) that of the standard legionary pay.