Meteorologists recorded a high of 46.6C (116F) in Lytton, British Columbia on Sunday, breaking what was said to be an 84-year-old record.
"Heat records are usually broken by decimals, like a tenth of a degree," environmental activist Ms Thunberg tweeted in response to record-breaking heat. "And not in June...This heat-wave is just getting started."
More high temperatures were recorded on Monday amid warnings the conditions could continue for several more days.
Experts say sweltering temperatures are being caused by a "heat dome" - a term given to an area of high pressure which lingers over an area for a prolonged period of time, and acts like the lid of a sauce pan trapping in heat.
Climate scientists say global warming has increased the frequency and intensity of extreme weather events such as heatwaves.
Seattle, Washington and Portland, Oregon, both recorded temperatures well above 37.8C (100F) on Monday.
The heat forced schools and businesses to close to protect workers and guests, including some places like outdoor pools and ice cream shops where people seek relief from the heat.
A number of Covid-19 testing sites and mobile vaccination units were also forced to shut.
Zeke Hausfather, a scientist at the climate-data nonprofit Berkeley Earth, California, said that the Pacific northwest has warmed by about 1.7C in the past half-century.
He said: "In a world without climate change, this still would have been a really extreme heatwave.
"This is worse than the same event would have been 50 years ago, and notably so."