The No. 8 seed found herself 5-2 down and behind a double break in the final set but managed to force a tiebreaker with No. 6 seed Tomljanovic and finally come out on top 6-2, 2-6, 7-6 (3).
The win marked a second title on the WTA Tour for the Ukrainian teenager, who will move into the top 40 in the world rankings for the first time Monday.
Yastremska came out firing in the final of the inaugural event in southern Thailand and raced into a 5-0 lead in the first set, before closing it out to 6-2.
The momentum shifted in the second and Australian Tomljanovic took control with consecutive breaks of serve early on as Yastremska twice fired down double faults on break point. Tomljanovic took advantage and improved her consistency, minimizing the unforced error count to close out the second set to 6-2.
Yastremska then broke at the beginning of the final set but was frustrated by the excellent defence of Tomljanovic, who reeled off five games to make it 5-2 and go within one of her first WTA Tour title.
But Yastremska rallied and secured two crucial breaks after a huge forehand down the line and an untimely double fault from Tomljanovic.
Both players held to make it into a tiebreak before Tomljanovic again double faulted at 3-3 to give Yastremska the advantage. The 18-year-old earned a match point then sealed the win with a cross-court backhand winner for the title.