The victory marks a significant milestone for the British No 5, who entered the tournament as a qualifier and secured her maiden WTA trophy at age 30. Dart recovered quickly from that miss to serve out the match to 15, falling on her back in delight when victory was sealed with an unreturned serve.
The British player has now won 19 of her 24 tour-level matches on hard courts since Wimbledon, bringing her overall tour record this season to 23 victories from 34 matches, the highest winning percentage of her career.
Dart dropped serve in the opening game of the match and faced early pressure after having to save a break point in her next service game against the fourth seed.
She earned two championship points in the final game, but Tagger saved the first with a big serve before Dart hit a mid-court forehand into the net on the second.
Dart advanced to the final after saving three match points in her semi-final against Sun Xinran.
In a tense deciding set, a sequence of five consecutive breaks of serve concluded when a gutsy Dart saved two break points to hold for a 5-3 lead.
Harriet Dart won her first tour-level title by defeating 18-year-old Austrian Tagger 4-6, 6-3, 6-4 at the Guangzhou Open. She recovered to level the match by claiming the first break point of the second set for 5-3.