Nabonidus (năbənī´dəs), d. 538? BC, last king of the Chaldaean dynasty of Babylonia. He was not of Nebuchadnezzar's family, and it is possible that he usurped the throne. He was absorbed in antiquarian and religious speculations, and he built temples while the state was left undefended. He was unpopular with both the priests and the people. When the Persian threat of Cyrus the Great grew strong, Nabonidus allied himself with Croesus of Lydia and Amasis II of Egypt, but to no avail. In 538 BC the kingdom fell to Cyrus with no resistance. Nabonidus' scholars preserved information valuable to modern archaeologists. Cuneiform records indicate that Belshazzar was Nabonidus' son and his coregent during the last years of Babylon.