Belshazzar was the son and crown prince of Nabonidus, the last king of the Neo-Babylonian Empire. Through his mother he might have been a grandson of Nebuchadnezzar II, though this is not certain and the claims to kinship with Nebuchadnezzar may have originated from royal propaganda.
Belshazzar played a pivotal role in the coup d'etat that overthrew the king Labashi-Marduk and brought Nabonidus to power in 556 BC. Since Belshazzar was the main beneficiary of the coup, through confiscating and inheriting Labashi-Marduk's estates and wealth, it is likely that he was the chief orchestrator. Through proclaiming his father as the new king, Belshazzar also made himself the first-in-line to the throne. As Nabonidus was relatively old at the time, Belshazzar could expect to become king within a few years.
Nabonidus was absent from Babylon from 553 BC to 543 or 542 BC, in self-imposed "exile" at Tayma in Arabia, for unknown reasons. For the duration of the decade-long absence of his father, Belshazzar served as regent in Babylon, a period which some historians characterize as a co-regency.