The Kazakhstani fourth seed broke to love in the opening game and cruised to the first set, breaking again to make it 5-2 and then holding to love.
Kostyuk had no answer and when her opponent broke serve once more in the first game of the second set, the unseeded Ukrainian cut a forlorn figure as she dropped to her knees.
Rybakina, who defeated world number one and winner of the last two titles Iga Swiatek (22) in the semi-finals, showed no mercy and Kostyuk forced her only break points over the course of the match when 3-2 down in the second set.
If there was to be a comeback, this was Kostyuk's chance, but she looked close to tears as she failed to take advantage of three break points, constantly looking in frustration at her coach in the crowd.
After holding serve, Rybakina broke again to make it 5-2 to leave her serving for the match. After hitting 10 aces in her win over Swiatek, she did not need any to dismiss Kostyuk.
Rybakina adds the Stuttgart title to her wins in the Brisbane International and the Abu Dhabi Open this year. For Kostyuk, this was her second final defeat of 2024 after she lost the decider of the San Diego Open in March.
"It's an amazing event and I hope to be back here every year, thank you," Kostyuk said.
"I want to say how incredibly proud I am to see so many Ukrainian flags out here this week."
