And the two players from Spain are the first men from somewhere other than the United States to sit at 1-2 since the tour introduced computerized rankings in 1973.
Alcaraz is a 19-year-old who rose to the top spot by winning the US Open last month for his first Grand Slam title, becoming the youngest man ever to be No. 1.
Nadal is a 36-year-old who has spent 209 weeks atop the rankings and owns a men's-record 22 major championships, including at the Australian Open in January and the French Open in June. He rose from No. 3 to No. 2 on Monday, swapping places with US Open runner-up Casper Ruud.
The last time players from the same country held Nos. 1 and 2 in the ATP rankings was in August 2000, when Americans Andre Agassi and Pete Sampras did it.
"Proud to share this historic moment with @RafaelNadal, the best player of all-time," Alcaraz wrote in Spanish on Twitter.
The only other changes in the ATP top 10 on Monday came with Jannik Sinner sliding from No. 10 to No. 12. That allowed Hubert Hurkacz to move up one spot to No. 10, and Taylor Fritz to shift one place up to a career-best No. 11.