Sinner broke late in the first set and then pulled away from De Minaur to complete a 7-5, 6-2 win -- his 13th in as many meetings with the Australian.
The Italian world number two has yet to drop a service game en route to the final. He will try to defend his title on Sunday against top-ranked Carlos Alcaraz or Felix Auger-Aliassime.
"It is the last event of the year and it is great to finish in this way," said Sinner.
"It was a very tough match, especially at the beginning of the first set. I felt like he was serving great, very precise.
"In the second set, I broke very early and then my level rose. I tried to be a bit more aggressive and it worked well but it was a tough match."
Sinner has won his last 18 sets played in Turin, where he beat Taylor Fritz in last year's final. The four-time Grand Slam champion is on a 30-match winning streak on indoor hard courts.
The 24-year-old Sinner hasn't dropped a set at the ATP Finals since losing the 2023 final to Novak Djokovic.
"Making the final for three consecutive years means a lot to me," said Sinner.
"It is a great atmosphere for me to play tennis and a great place for me to close this beautiful season. Tomorrow I will enjoy and try my best to get the best possible result."
De Minaur fought off two break points in the opening game of the semi-final but then failed to convert any of the three he carved out in Sinner's first service game.
The Australian resisted again when Sinner threatened to break in the seventh and ninth games, but the second seed eventually struck the key blow at 5-5 and snatched the first set.
Sinner surged 4-0 ahead in the second set before wrapping up his ninth straight win at the tournament, becoming the youngest player since Lleyton Hewitt in 2004 to reach three finals at the season-ending event.
If Sinner faces Alcaraz in the final, it will be their sixth meeting of the season. Alcaraz has won four of the five previous matches, triumphing in the French Open and US Open finals.