After an even start between the two, the sixth seeded Ukrainian began to get the better of Boulter, stringing multiple games together and rapidly closing in on the opening set.
Boulter fought back to even the set at 5-5, but Kostyuk regained the momentum to wrap up the first set 7-5.
Boulter then found her groove in the second, dominating on her first serve to clinch the set 6-2 and force a deciding third set.
The Briton continued her ascendancy into the third, claiming the first break point of the set en route to opening up a 3-1 advantage.
Boulter broke again in the seventh game and stormed home from there to close out the victory in two hours and 13 minutes.
The 27-year-old from Leicester was watched by boyfriend Alex De Minaur, who scheduled an early-morning flight after retaining his title in Acapulco.
Boulter thanked her Australian partner in her post-match comments.
“I want to say a small special thank you to my boyfriend,” she said.
“He finished last night at midnight and I really want to embarrass him.
“He got on a 4.15 taxi this morning and six o’clock flight to be here today, so I do appreciate it.”
She continued: “This week has been very, very special for so many different reasons. This one is pretty amazing, I’ve worked very hard for it, I played some incredible tennis all week.
“Today was a complete battle, with myself as well, because I was a little bit nervous. But I managed to get over the line, and that I’m very proud of.
“A lot of it was about me staying as tough as I possibly could mentally, and I managed to keep my cool and actually kind of went within myself and calmed myself down a lot. I think that really helped me, and then I started to relax and play through shots a little bit more.”
Both women picked up their first WTA Tour titles last year in breakout 2023 seasons.
Boulter claimed her maiden championship on grass in Nottingham last summer, while Kostyuk found victory in Austin.
Boulter has had a flying start to the 2024 season and the victory over Kostyuk guarantees she will break into the top 30 for the first time.
