The 14-year-old left-hander edged past his Indian opponent in straight sets, 7-6, 7-5, in a tight contest that featured extended rallies and a first-set tie-break.
The tournament was held in China's Hainan Province.
Tien Anh, born in 2011, is known for his powerful and spinning left-handed serve.
His victory over Dahiya — the tournament favorite — marks a significant milestone for Vietnamese tennis.
The result secures Tien Anh a wild card spot for the Roland Garros Junior Series event in Japan from October 13–17, where he will compete for a place in the 2026 main draw in Paris.
Earlier in the tournament, Tien Anh upset No.2 seed Ranvir Singh, also from India, in the opening round with a 6-3, 5-7, 6-2 win.
Le Tien Anh at Roland Garros Junior 2025. Photo: VTF
He followed with a hard-fought 5-7, 7-6, 6-2 victory over South Korea's Bang John Hyun Gyum and a straight-sets win against Lai Tsz Ho to top Group F.
In the semi-finals, he defeated Liu Ian of Chinese Taipei 6-3, 6-1 to reach the final.
Tien Anh becomes the first Vietnamese player to win a regional qualifier of the ITF-sanctioned Roland Garros Junior Series.
Former national standout Ly Hoang Nam competed at Roland Garros Junior in 2014 and 2015 but did not win a title.
"I'm very happy to win and will now focus on the upcoming event in Japan," Tien Anh said after the match.
