Reutimann took the lead when he gambled and didn't pit with the leaders during a caution for rain on lap 222 of the 400-lap race. His hope was that it would rain so long, racing would not resume.
After five laps as the leader run under caution, NASCAR called the cars to pit road. Reutimann nervously paced around his covered race car for the next 2 hours as NASCAR waited out the weather.
The rain never stopped, and the race was called a full day after its scheduled start.
"When you envision winning your first Sprint Cup race, you envision it different," said a soaking-wet Reutimann. "But it's so hard, we'll take it any way we can. We got this one today, hopefully the next one, we'll earn it."
The race was scheduled for Sunday but rain moved it to Memorial Day for the first time in its 50-year history. The showers lingered all day, and the race was stopped three different times for rain and once for a national moment of remembrance.
It was the first win in 75 Cup starts for Reutimann, who drives for Michael Waltrip Racing. He was joined by his car owner midway through the final delay, a wait Waltrip has done before: He earned his second Daytona 500 title after a lengthy rain delay shortened the 2003 event.
Pole-sitter Ryan Newman finished second and Robby Gordon was third as the top-three finishers all stayed on the track for position during the last pit stops.
Carl Edwards, who had changed into street clothes by the time the race was called, finished fourth, followed by Brian Vickers and Kyle Busch.
Busch led a race-high 173 laps but was denied a win because of rain for the second time in three days. He led 143 laps in Saturday night's Nationwide Series race, lost the lead during a round of pit stops, and never had a chance to reclaim it when the race was shortened by 45 miles because of rain.
It's the second time this season one of NASCAR's crown jewel events was ruined by rain. The season-opening Daytona 500 was shortened by 48 laps and Matt Kenseth was declared the winner after a short delay.