The 15th-ranked Garcia, who lost to Halep the two previous times they played, will make her top-10 debut when the new rankings come out on Monday.
"She played amazing tennis," Halep said of Garcia. "She deserved to win today. She was better."
For Halep, the loss was particularly disappointing as she was unable to back up her guaranteed debut in the No. 1 ranking on Monday. She secured the top spot on Saturday with her semifinal victory over French Open champion Jelena Ostapenko.
Halep's best opportunity to rebound against Garcia ended when she failed to make good on any of the nine break points she had at 3-3 in the second set.
"For sure it definitely turned the second set," Garcia said. "This game was definitely very important."
In the second-set tiebreaker, the unseeded Garcia jumped out to a 4-1 lead, and on her first match point at 6-3 with Halep serving, the Romanian netted a forehand. Garcia fell to her knees in celebration.
She is enjoying an 11-match winning streak, having captured her first title of the season at Wuhan last week. This is the second time she's won a career-best 11 straight matches.
"It was such an amazing two weeks," Garcia said. "It went so fast."
The Frenchwoman saved a match point in her three-set quarterfinal victory over third-seeded Elina Svitolina of Ukraine.