Nadal has staged a spectacular comeback after being sidelined for seven months with a knee injury from June last year.
Since returning in February, he has reached the final in all of the nine events he has played, winning seven titles including Indian Wells and the Madrid and Rome Masters Series.
The 27-year-old is fifth in the world rankings, but is first in the Race standings that are based on the calendar year's results.
He has already guaranteed a top-eight finish and a place at the end-of-season showpiece event in London.
"Nadal has now qualified for the ATP World Tour Finals nine straight years," the ATP said.
The Spaniard, who has 12 Grand Slam titles to his name, has never won the Tour Finals, with his best performance being a final defeat to Roger Federer in 2010.
Nadal missed last year's event in London due to the knee injury that also forced him out of the London Olympics, the US Open and this year's Australian Open.
He thrashed fellow Spaniard David Ferrer on Sunday to become the first player to win the same Slam eight times.
The World Tour finals will take place in London from November 4-11.
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