Djokovic took 1 hour, 43 minutes to win 6-3, 6-3 as the Serbian continues to break records at the age of 36.
He entered the match tied with Roger Federer with six titles at the season-ending tournament for the year’s top eight players.
It was his fourth win over Sinner, who had recorded his first-ever victory against Djokovic in the group stage in Turin and was the first Italian player to reach the final.
But Djokovic was in imperious form Sunday and won 14 straight points from the end of the first set to the third game of the second to leave him firmly in control and subdue the Turin crowd.
Djokovic had already secured the year-end No. 1 ranking for a record-extending eighth time by winning his opening match at the ATP Finals.
After this tournament, Djokovic will become the first player to hold the No. 1 ranking for 400 weeks, with Roger Federer at 310 the only other man to eclipse the 300-week mark.
It was also only the second time in the last 15 years that a player has made the finals of all four Grand Slam tournaments and the ATP Finals. The only previous occasion was when Djokovic did it in 2015.