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.
Experts say climate change is expected to increase the frequency of extreme weather events, such as heatwaves, however linking any single event to global warming is complicated.
As the climate changes, there could be an increase in the number of deaths from floods, storms and heatwaves, experts say.
NWS said that even hotter temperatures were forecast on Sunday and Monday throughout the Pacific Northwest and Northern Great Basin.
It warned of "several more days of dangerous heat across the northwest corner of the country as well as parts of western Nevada and California".
Despite the warnings many people have been enjoying the sunshine, with lakes busy and pools running at full capacity.