Year 142 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Calvus and Servilianus (or, less frequently, year 612 Ab urbe condita). The denomination 142 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

• Diodotus Tryphon seizes the throne of the Seleucid Empire.

• The first stone bridge over the Tiber river is completed.

• Simon Maccabaeus succeeds his brother Jonathan as High Priest of Judea until 135 BC.

• Ptolemy IX, Egyptian pharaoh (d. 81 BC)