The Spaniard, who has risen from ninth at the start of the year via French and US Open victories, advanced 7-5 6-3 in one hour and 48 minutes.
“The most important moment of the year was the clay-court season and winning a 10th Roland Garros title here,” he told the crowd in his on-court interview.
Nadal, 31, will play Uruguay’s world number 36 Pablo Cuevas in the last 16.
The 16-time Grand Slam champion did not have it all his own way in a see-sawing first set in which he and Chung exchanged two breaks apiece before Nadal broke to love in the final game.
South Korea’s Chung, 21, held his own in the second set but again crumbled in the latter stages, surrendering to love to give up the solitary break.
Nadal is 1,460 points clear of Swiss world number two Roger Federer in the world rankings and victory over Chung ensured him an unassailable lead before the World Tour Finals in London later this month.
Nadal, who also finished 2008, 2010 and 2013 at the top of the rankings, is the oldest player to finish a campaign at world number one since they were introduced in 1973.
Andy Murray, who was 2016’s season-end number one, is third with 20-year-old Alexander Zverev fourth.