Jabeur lost out to Elena Rybakina in the Wimbledon final earlier this summer but has the chance to go one better after a 6-1 6-3 victory against France's Garcia.
Jabeur, the first African woman to reach the last four in New York in the open era, lay on the court in celebration.
The Tunisian said: "It feels amazing. After Wimbledon there was a lot of pressure on me. I'm really proving that I can back up my results. The hard court season started a little bit bad but I'm really happy I can make it to the finals here."
There was a brief moment of silence to honour the Queen before the match began on Arthur Ashe Stadium.
Jabeur and Garcia have been playing each other since they were juniors, with Jabeur coming out on top each time.
Garcia has been the woman in form this summer, winning three titles and climbing back towards the top 10, but it was clear from the start that the 28-year-old was nervous in the biggest match of her career.
Shots that had been flying inside the lines throughout the fortnight were missing, while Jabeur was able to neutralise the pace of Garcia in a way that none of her previous opponents had.
The opening set lasted just 23 minutes, with Jabeur sending down six aces, while Garcia made 14 unforced errors.
It was the first set Garcia had dropped all tournament and she tried to dig in at the start of the second but she was still giving away too many free points while Jabeur was in her element on the big stage.
The Garcia serve, such a strength this fortnight, was being taken apart, and her big night was quickly becoming a nightmare.
She at least rallied to force Jabeur to serve it out but the fifth seed was not to be denied and converted her first match point.