The US National Weather Service (NWS) has issued excessive heat warnings and watches across nearly all of Washington and Oregon state. Parts of California and Idaho are also affected.
Multnomah county, in Oregon, has warned of "life-threatening" heat.
Some cities have opened cooling centres, where residents can escape the heat in air-conditioned buildings.
The soaring temperatures are due to a dome of high pressure hovering over northwestern United States and Canada.