Mercurius (Hungarian: Merkúr)^[1] was a Hungarian distinguished nobleman and perhaps the first known voivode of Transylvania, who held the office during the reign of Coloman, King of Hungary.^[1] Two royal charters issued in 1111 and 1113 mention Mercurius as "princeps Ultrasilvanus", but he may have been only an important landowner in Transylvania without holding any specific office.^[2] The title voivode was first documented^[where?] specifically in 1199.^[3]

A source from 1097^[which?] also mentions a "Mercurio comes Bellegratae", which means "count (ispán) of Fehér", and this record may point to the same person.^[2][4][5]

• Curta, Florin (2006). Southeastern Europe in the Middle Ages, 500-1250. Cambridge University Press. ISBN 978-0-521-89452-4.
• Makkai, László (2001). Transylvania in the medieval Hungarian kingdom (896-1526), In: Béla Köpeczi, HISTORY OF TRANSYLVANIA Volume I. From the Beginnings to 1606, Columbia University Press, New York, 2001, ISBN 0-88033-479-7
• (in Hungarian) Markó, László: A magyar állam főméltóságai Szent Istvántól napjainkig – Életrajzi Lexikon (The High Officers of the Hungarian State from Saint Stephen to the Present Days – A Biographical Encyclopedia) (2nd edition); Helikon Kiadó Kft., 2006, Budapest; ISBN 963-547-085-1.
• (in Hungarian) Zsoldos, Attila (2011). Magyarország világi archontológiája, 1000–1301 ("Secular Archontology of Hungary, 1000–1301"). História, MTA Történettudományi Intézete. Budapest. ISBN 978-963-9627-38-3