The News Agency of Nigeria reports that it is his fourth time of finishing atop the rankings, adding to 2008, 2010 and 2013.
"For me it has much more value (to) finish the year than being number one in the middle of the season".
"I'm very, very happy for everything", said Nadal in his on-court interview.
Roger Federer's withdrawal from the tournament in order to rest ahead of the ATP Finals meant Nadal needed only one win to guarantee his great rival could not catch him.
Nadal, who reclaimed the top ranking in August for the first time in three years, is due to start at Bercy on Wednesday against South Korea's Hyeon Chung, who opened on Monday with a 6-0, 6-2 first-round defeat of German Mischa Zverev.
"It's something that means a lot to me". It's been an wonderful year, ' he said after the match.
'I'm happy to achieve the year-end world No. 1 here in Paris'.
This has been a remarkable season for Nadal, who bounced back from wrist problems past year to win his 15th and 16th grand slam titles at the French Open and US Open.
Nadal, who has never won the tournament, was a break behind in the opening set before stepping up a gear to win four games in a row.
Another home player, Lucas Pouille, kept his slim hopes of reaching London alive by seeing off Spaniard Feliciano Lopez 6-3, 6-4, but US Open runner-up Kevin Anderson's chances ended with a 5-7, 6-4, 7-5 loss to Fernando Verdasco.
But the 31-year-old Spaniard - who is also trying to balance the delicate political situation in his native country with his status as an worldwide sportsman - refused to hype up his potential coronation.
But he responded with a truly a remarkable comeback year to become the oldest man to finish a season at the top of the rankings at the age of 31, after a year that saw him win the French and US Opens.
Nadal's results in Paris will give a good idea for how suited he now is to make a run at the biggest tournament he has never won in his career-the World Tour Finals in London in two weeks.