A valley near Salem, the city where Melchizedek was king. This is where Abraham met the king of Sodom. Abraham refused an offer from the king of Sodom that would have been morally wrong (Genesis 14:17). The valley is also called Shaveh.
If Salem is the same place as Jerusalem, the "king's valley" is probably either the Kidron Valley or the valley of Hinnom. These are two valleys near ancient Jerusalem.
This same valley was also the place where Absalom (King David's son) built a stone pillar as a monument to himself (2 Samuel 18:18).