Reis is a common surname in the Portuguese language, namely in Portugal and Brazil.
It was originally a Christian devotional family name of the Middle Ages, probably due to the Portuguese name for the Biblical Magi, the Reis Magos (the Magi Kings).
Sometimes the surname is dos Reis (of the Kings).
The Reis etymology is probably from the Latin Rex ("King"), and it is noticeable that it has relations with the German Reich ("Kingdom, Empire"), and the Dutch Rijk (also "Kingdom, Empire"), the Germanic names Rick, Rich, Richard, etc.
Reis is also a common surname in the German language (where its alternative meaning is "rice"), a historically famous epithet derived from military rank in Turkey (e.g., Piri Reis), and was also used by some European Jews.