Temperatures from the Midwest to the East Coast dropped from highs above 38 degrees Celsius down to mid 30s.
Cooler air swept southward in the eastern half of the country, bringing down some temperatures by 15 or more degrees from Saturday's highs, which topped 38 C in cities including Philadelphia, Washington, St. Louis, Indianapolis and Louisville, Kentucky.
For many areas, the cooler temperatures were ushered in by thunderstorms that knocked out power to thousands. In New Jersey, a line of strong, fast-moving storms knocked out power to nearly 70,000 on Saturday night.
A 4-month-old girl died and a 16-month-old girl was hospitalized in suburban Indianapolis after both were found trapped in cars during 40.5 C heat Saturday. Deaths have also been reported by authorities in Illinois, Maryland, Ohio, Pennsylvania, Tennessee and Wisconsin.
The heat caused highways to buckle in Illinois and Wisconsin, officials said. In Maryland, investigators said heat likely caused rails to kink and led a commuter train to partially derail Friday. No one was injured.
To stay cool, Americans tried familiar solutions — dipping into the pool, going to the movies and riding subways just to be in air conditioning.
Even the beach offered no respite. Atlantic City, New Jersey, home of the famed boardwalk, set a temperature record Saturday of 38 C.
If Americans ventured outside to do anything, they did it early. But even then, the heat was stifling.
"It was baking on the 18th green," said golfer Zeb Rogerson, who teed off at 6 a.m. at an Alexandria, Virginia, golf course but was sweltering by the end of his round.
In South Bend, Indiana, serious kayakers took to the East Race Waterway, a 579-metre-long man-made whitewater course near downtown.
"A lot of times I'll roll over just to cool off," said Robert Henry of Carmel, just north of Indianapolis. "The biggest challenge is walking coming back up carrying a kayak three-eighths of a mile in this heat."
In Manhattan, customers who stepped in to see Jiro Dreams of Sushi at an IFC movie theatre were there for more than entertainment.
"Of course we came to cool off!" said John Villanova, a writer who was on his second sweaty T-shirt of the day and expecting to change again by evening.
He said that earlier, he rode a Manhattan subway back and forth for a half an hour, with no destination in mind "because it really keeps you cool."
In Chicago, street magician Jeremy Pitt-Payne said he has been working throughout the three-day stretch of triple-digit temperatures, but acknowledged that he might doff the Union Jack leather vest by the end of the day, even though it's part of his British magician character along with the black top hat.
His trick for beating the heat? He starts his shows at 2 p.m., "when the Trump Tower is gracious enough to block out the sun" along his stretch of sidewalk.to block out the sun" along his stretch of sidewalk.