In the biblical Book of Genesis, Er (Hebrew: עֵר, Modern: Er, Tiberian: ʻĒr "watcher"; Greek: Ἤρ) was the eldest son of Judah and his Canaanite wife, the daughter of Shuah. He is described as marrying Tamar. According to the text, "the LORD slew" Er because he was wicked, although it does not give any further details. According to Rashi, he, like Onan, purposely tried to keep Tamar from conceiving, in his case because he was afraid of spoiling her beauty. This Er had two brothers, Onan and Shelah.
Some modern bible critics interpret the story of Er as an eponymous aetiological myth to explain fluctuations in the constituency of the tribe of Judah, with the abrupt death of Er reflecting the death of a clan; the presence of an Er as a descendant of Shelah, in the Book of Chronicles, suggests that Er was in reality the name of a clan that was originally equal in status to the Shelah clan, but was later subsumed by it. The brother - Onan - may represent an Edomite clan named Onam, who are mentioned in an Edomite genealogy in Genesis.
These critics have also argued that the Tamar narrative, of which the description of Er is a part, secondarily aims to either assert the institution of levirate marriage, or present an aetiological myth for its origin; Er's role in the narrative would thus be as the background to the main plot, his death being the reason for levirate marriage to become an issue. John Emerton, Regius Professor of Hebrew at the University of Cambridge, regards the evidence for this as inconclusive, though classical rabbinical writers argued that this narrative does concern the origin of levirate marriage.