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]