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[1][2] and were responsible for getting the watchwords from the commander and seeing that they were kept safe.
There was one tesserarius to each centuria (Wilkes, 1972).
They held a position similar to that of a first sergeant of a company 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.